Debt Disclosure [Abstract]
Schedule of Long-term Debt Instruments [Table]
Schedule of Long-term Debt Instruments [Table]
Debt Instrument [Axis]
Debt Instrument [Axis]
Debt Instrument, Name [Domain]
Debt Instrument, Name [Domain]
ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member]
ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member]
ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member]
Debt Instrument [Line Items]
Debt Instrument [Line Items]
Short-Term Debt - Principal
Debt, Current, Gross
Amount of short-term debt and current maturities of long-term debt before deduction of unamortized discount or premium and unamortized debt issuance costs.
Short-Term Debt - Unamortized Debt Issuance Costs
Unamortized Debt Issuance Expense, Debt, Current
The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of short-term debt or the current maturities of long-term debt.
Short-Term Debt - Total
Debt, Current
Long-term Debt - Principal
Long-term Debt, Gross
Long-Term Debt - Unamortized Debt Issuance Costs
Unamortized Debt Issuance Expense, Long-Term Debt Excluding Current Maturities
The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of long-term debt excluding current maturities.
Long-Term Debt - Total
Long-term Debt, Excluding Current Maturities
Total Debt - Principal
Debt, Long-Term and Short-Term, Combined Amount, Gross
Represents the aggregate of total long-term debt, including current maturities and short-term debt, before deduction of unamortized discount or premium and unamortized debt issuance expense.
Total Debt - Unamortized Debt Issuance Costs
Unamortized Debt Issuance Expense
Total Debt - Total
Debt, Long-term and Short-term, Combined Amount
Long-Term Debt Issued
Proceeds from Issuance of First Mortgage Bond
Expected Proceeds from Future Issuance of Senior Unsecured Notes
Expected Proceeds from Future Issuance of Senior Unsecured Notes
The expected future cash inflow from amounts received from a future unsecured long-term debt instrument issuance.
Expected Future Interest Rate
Future Debt Instrument, Interest Rate, Stated Percentage
Contractual interest rate for funds expected to be borrowed, under the debt agreement.
Equity [Abstract]
Reclassifications Out of Accumulated Other Comprehensive Income (Loss) [Text Block]
Disclosure of Reclassification Amount [Text Block]
Equity Method Investments and Joint Ventures [Abstract]
Investment in ATC [Text Block]
Equity Method Investments and Joint Ventures Disclosure [Text Block]
Regulated Operations [Abstract]
Regulatory Assets and Liabilities [Table Text Block]
Schedule of Regulatory Assets and Liabilities [Text Block]
Statement of Financial Position [Abstract]
Accounts Receivable [Abstract]
Accounts Receivable, Net, Current [Abstract]
Accounts Receivable, Allowance
Allowance for Doubtful Accounts Receivable, Current
Common Stock [Abstract]
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Common Stock, Par Value
Common Stock, No Par Value
Common Stock, Shares Authorized
Common Stock, Shares Authorized
Common Stock, Shares Outstanding
Common Stock, Shares, Outstanding
Common Stock, Shares Issued
Common Stock, Shares, Issued
Commitments and Contingencies Disclosure [Abstract]
Commitments, Guarantees and Contingencies [Text Block]
Commitments Contingencies and Guarantees [Text Block]
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Inventories – Net [Policy Text Block]
Inventory, Policy [Policy Text Block]
Subsequent Events [Policy Text Block]
Subsequent Events, Policy [Policy Text Block]
New Accounting Standards [Policy Text Block]
New Accounting Pronouncements, Policy [Policy Text Block]
Land Inventory [Policy Text Block]
Real Estate Held for Development and Sale, Policy [Policy Text Block]
Available-for-sale Investments [Policy Text Block]
Marketable Securities, Available-for-sale Securities, Policy [Policy Text Block]
Acquisitions [Policy Text Block]
Business Combinations Policy [Policy Text Block]
Fair Value Measurement [Policy Text Block]
Fair Value Measurement, Policy [Policy Text Block]
North Dakota Investment Tax Credits [Policy Text Block]
Regulatory Income Taxes, Policy [Policy Text Block]
Regulatory Assets and Liabilities [Policy Text Block]
Public Utilities, Policy [Policy Text Block]
Equity Method Investments [Policy Text Block]
Equity Method Investments, Policy [Policy Text Block]
Income Tax [Policy Text Block]
Income Tax, Policy [Policy Text Block]
Earnings Per Share [Policy Text Block]
Earnings Per Share, Policy [Policy Text Block]
Power Purchase Agreements [Policy Text Block]
Consolidation, Variable Interest Entity, Policy [Policy Text Block]
Environmental Accruals [Policy Text Block]
Environmental Costs, Policy [Policy Text Block]
Business Segments [Policy Text Block]
Segment Reporting, Policy [Policy Text Block]
Revision of Prior Balance Sheet [Table]
Schedule of Quantifying Prior Year Misstatement Corrected in Current Year Financial Statements [Table]
Balance Sheet Location [Axis]
Balance Sheet Location [Axis]
Balance Sheet Location [Domain]
Balance Sheet Location [Domain]
Regulatory Assets [Member]
Regulatory Assets [Member]
Regulatory Assets [Member]
Deferred Income Taxes [Member]
Deferred Income Taxes [Member]
Deferred Income Taxes [Member]
Revision of Prior Balance Sheet [Line Items]
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Immaterial Revision
Quantifying Misstatement in Current Year Financial Statements, Amount
Fair Value Disclosures [Abstract]
Recurring Fair Value Measures [Table]
Fair Value Measurements, Recurring and Nonrecurring [Table]
Measurement Frequency [Axis]
Fair Value, Measurement Frequency [Domain]
Recurring Fair Value Measurements [Member]
Fair Value, Measurements, Recurring [Member]
Fair Value, Hierarchy [Axis]
Fair Value Hierarchy [Domain]
Level 1 [Member]
Fair Value, Inputs, Level 1 [Member]
Level 2 [Member]
Fair Value, Inputs, Level 2 [Member]
Level 3 [Member]
Fair Value, Inputs, Level 3 [Member]
Contingent Consideration [Axis]
Contingent Consideration [Axis]
Contingent Consideration [Axis]
Contingent Consideration [Domain]
Contingent Consideration [Domain]
[Domain] for Contingent Consideration [Axis]
Accretion [Member]
Accretion [Member]
Accretion expense [Member]
Payments [Member]
Payments [Member]
Payments made that decrease contingent consideration liability [Member]
Changes in Cash Flow Projections [Member]
Changes in Cash Flow Projections [Member]
Changes in Cash Flow Projections [Member]
Recurring Fair Value Measures [Abstract]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets [Abstract]
Assets, Fair Value Disclosure [Abstract]
Investments [Abstract]
Investments [Abstract]
Available-for-sale – Equity Securities
Available-for-sale Securities, Equity Securities, Noncurrent
Available-for-sale – Corporate and Governmental Debt Securities
Available-for-sale Securities, Debt Securities, Noncurrent
Cash Equivalents
Other Restricted Assets, Noncurrent
Total Fair Value of Assets
Assets, Fair Value Disclosure, Recurring
Liabilities [Abstract]
Liabilities, Fair Value Disclosure [Abstract]
Deferred Compensation
Deferred Compensation, Fair Value Disclosure
This element represents the portion of the balance sheet assertion, Deferred Compensation, valued at fair value by the entity.
U.S. Water Services Contingent Consideration
Business Combination, Contingent Consideration, Liability
Total Fair Value of Liabilities
Liabilities, Fair Value Disclosure, Recurring
Total Net Fair Value of Assets (Liabilities)
Fair Value, Net Asset (Liability)
Fair Value Hierarchy Transfers, All Levels
Fair Value Transfers Between Levels, All Levels
The amount of transfers in and out of levels 1, 2, and 3 of the fair value hierarchy during the period.
Activity in Level 3 [Roll Forward]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value
Activity in Level 3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease)
Ending Balance
U.S. Water Services Contingent Consideration, Current
Business Combination, Contingent Consideration, Liability, Current
Income Tax Disclosure [Abstract]
Income Tax Expense [Text Block]
Income Tax Disclosure [Text Block]
New Accounting Standards[Table]
New Accounting Pronouncements or Change in Accounting Principle [Table]
Adjustments for New Accounting Pronouncements [Axis]
Adjustments for New Accounting Pronouncements [Axis]
Type of Adoption [Domain]
Type of Adoption [Domain]
Accounting Standards Update 2016-09 [Member]
Accounting Standards Update 2016-09 [Member]
Accounting Standards Update 2016-09 [Member]
Income Statement Location [Axis]
Income Statement Location [Axis]
Income Statement Location [Domain]
Income Statement Location [Domain]
Income Tax Expense (Benefit) [Member]
Income Tax Expense [Member]
Income Tax Expense [Member]
Range [Axis]
Range [Axis]
Range [Domain]
Range [Domain]
Maximum [Member]
Maximum [Member]
New Accounting Standards [Line Items]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effect of the Adoption
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification
Inventories [Table Text Block]
Schedule of Inventory, Current [Table Text Block]
Prepayments and Other Current Assets [Table Text Block]
Schedule of Other Current Assets [Table Text Block]
Other Non-Current Assets [Table Text Block]
Schedule of Other Assets, Noncurrent [Table Text Block]
Other Current Liabilities [Table Text Block]
Other Current Liabilities [Table Text Block]
Other Non-Current Liabilities [Table Text Block]
Other Noncurrent Liabilities [Table Text Block]
Supplemental Statement of Cash Flows Information [Table Text Block]
Schedule of Cash Flow, Supplemental Disclosures [Table Text Block]
Business Combinations [Abstract]
Schedule of Business Acquisitions [Table]
Schedule of Business Acquisitions, by Acquisition [Table]
Business Acquisition [Axis]
Business Acquisition [Axis]
Business Acquisition, Acquiree [Domain]
Business Acquisition, Acquiree [Domain]
Water & Energy Systems Technology [Member]
Water & Energy Systems Technology of Nevada, Inc. [Member]
Water & Energy Systems Technology of Nevada, Inc. [Member]
Business Acquisition [Line Items]
Business Acquisition [Line Items]
Percentage of Voting Interests Acquired
Business Acquisition, Percentage of Voting Interests Acquired
Name of Acquired Entity
Business Acquisition, Name of Acquired Entity
Total Consideration
Business Combination, Consideration Transferred
Cash Payment to Acquire Business
Payments to Acquire Businesses, Gross
Payment Due in April 2018
Business Combination, Consideration Transferred, Liabilities Incurred
Assets Acquired [Abstract]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract]
Cash and Cash Equivalents
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents
Other Current Assets
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets
Customer Relationships
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill
Goodwill
Goodwill
Other Non-Current Assets
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets
Total Assets Acquired
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets
Liabilities Assumed [Abstract]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract]
Current Liabilities
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities
Non-Current Liabilities
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities
Total Liabilities Assumed
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities
Net Identifiable Assets Acquired
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net
Tax Deductible Goodwill
Business Acquisition, Goodwill, Expected Tax Deductible Amount
Transmission [Table]
Transmission [Table]
Presents detailed information about transmission investments.
Power Purchase Agreements [Axis]
Long-term Contract for Purchase of Electric Power [Axis]
Power Purchase Agreements [Domain]
Long-term Contract for Purchase of Electric Power [Domain]
Manitoba Hydro PPA (expires 2035) [Member]
Manitoba Hydro PPA (expires 2035) [Member]
Manitoba Hydro PPA (expires 2035) [Member]
Counterparty Name [Axis]
Counterparty Name [Axis]
Counterparty Name [Domain]
Counterparty Name [Domain]
Manitoba Hydro [Member]
Manitoba Hydro [Member]
Manitoba Hydro [Member]
Legal Entity [Axis]
Legal Entity [Axis]
Entity [Domain]
Entity [Domain]
Minnesota Power [Member]
Minnesota Power [Member]
Minnesota Power [Member]
Transmission [Axis]
Information about each transmission investment.
Transmission [Domain]
Name of transmission investment.
Great Northern Transmission Line [Member]
Great Northern Transmission Line [Member]
Great Northern Transmission Line between Manitoba and Minnesota's Iron Range [Member]
Minimum [Member]
Minimum [Member]
Transmission [Line Items]
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
Output Being Purchased (MW)
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MW)
Share of plant output being purchased in megawatts under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.
Transmission Line Length (Miles)
Transmission Line Length (Miles)
Length of a transmission line in miles.
Transmission Line Capacity (kV)
Transmission Line Capacity (kV)
Capacity of a transmission line in kilovolts.
Total Project Cost in the U.S.
Estimated Capital Expenditures, Including Past Expenditures
The estimated amount of certain capital expenditures related to environmental compliance, regulatory matters, or other commitments and/or contingencies. This estimate includes expenditures expected in future periods and expenditures made through the end of the current period.
Total Project Costs Incurred to Date
Capital Cost Spent to Date
Capital cost spent to date for project.
Project Costs Recovered from Counterparty
Capital Costs Spent To Date, Recovered
Capital Costs Spent To Date, Recovered
Compensation and Retirement Disclosure [Abstract]
Pension and Other Postretirement Benefit Plans [Table]
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table]
Pension and Other Postretirement Benefit Plans [Axis]
Defined Benefit Plans and Other Postretirement Benefit Plans [Axis]
Pension and Other Postretirement Benefit Plans [Domain]
Defined Benefit Plan and Other Postretirement Benefit Plan [Domain]
Pension [Member]
Pension Plan [Member]
Other Postretirement [Member]
Other Postretirement Benefit Plan [Member]
Pension and Other Postretirement Benefit Plans [Line Items]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Components of Net Periodic Benefit Expense (Income) [Abstract]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service Cost
Defined Benefit Plan, Service Cost
Interest Cost
Defined Benefit Plan, Interest Cost
Expected Return on Plan Assets
Defined Benefit Plan, Expected Return on Plan Assets
Amortization of Prior Service Credits
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)
Amortization of Net Loss
Defined Benefit Plan, Amortization of Gains (Losses)
Net Periodic Benefit Expense (Income)
Defined Benefit Plan, Net Periodic Benefit Cost
Employer Contributions to Defined Benefit Pension Plan - Cash
Pension Contributions
Employer Contributions to Defined Benefit Pension Plan - Common Stock Value
Stock Issued During Period, Value, Employee Benefit Plan
Employer Contributions to Defined Benefit Plan
Defined Benefit Plan, Contributions by Employer
Expected Future Employer Contributions in 2017
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year
Acquisition of Non-Controlling Interest [Table]
Noncontrolling Interest [Table]
ALLETE Clean Energy [Member]
ALLETE Clean Energy [Member]
ALLETE Clean Energy [Member]
Acquisition of Non-Controlling Interest [Line Items]
Noncontrolling Interest [Line Items]
Payment to Acquire Non-Controlling Interest
Payments to Acquire Additional Interest in Subsidiaries
Gain or Loss in Net Income or Other Comprehensive Income
Other Nonoperating Gains (Losses)
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets [Text Block]
Goodwill and Intangible Assets Disclosure [Text Block]
Document and Entity Information [Abstract]
Document and Entity Information [Abstract]
Document and Entity Information [Table]
Document Information [Table]
Document and Entity Information, Document [Axis]
Document Information, Document [Axis]
Document [Domain]
Document [Domain]
Document and Entity Information [Line Items]
Document Information [Line Items]
Entity Registrant Name
Entity Registrant Name
Entity Central Index Key
Entity Central Index Key
Entity Tax Identification Number
Entity Tax Identification Number
Current Fiscal Year End Date
Current Fiscal Year End Date
Entity Well-known Seasoned Issuer
Entity Well-known Seasoned Issuer
Entity Voluntary Filers
Entity Voluntary Filers
Entity Current Reporting Status
Entity Current Reporting Status
Entity Filer Category
Entity Filer Category
Entity Common Stock, Shares Outstanding
Entity Common Stock, Shares Outstanding
Document Fiscal Year Focus
Document Fiscal Year Focus
Document Fiscal Period Focus
Document Fiscal Period Focus
Document Type
Document Type
Amendment Flag
Amendment Flag
Document Period End Date
Document Period End Date
Unrecorded Unconditional Purchase Obligation [Table]
Coal, Rail and Shipping Contracts [Axis]
Unrecorded Unconditional Purchase Obligation by Category of Item Purchased [Axis]
Coal, Rail and Shipping Contracts [Domain]
Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain]
Coal Supply and Transportation Agreements [Member]
Coal and Transportation [Member]
Coal and Transportation [Member]
Coal, Rail and Shipping Contracts [Line Items]
Unrecorded Unconditional Purchase Obligation [Line Items]
Minimum Annual Payment Obligation for Remainder of 2017
Unrecorded Unconditional Purchase Obligation, Due in Remainder of Fiscal Year
Minimum Annual Payment Obligation in 2018
Unrecorded Unconditional Purchase Obligation, Due within Two Years
Minimum Annual Payment Obligation in 2019
Unrecorded Unconditional Purchase Obligation, Due within Three Years
Minimum Annual Payment Obligation in 2020
Unrecorded Unconditional Purchase Obligation, Due within Four Years
Minimum Annual Payment Obligation in 2021
Unrecorded Unconditional Purchase Obligation, Due within Five Years
Minimum Annual Payment Obligation Thereafter
Unrecorded Unconditional Purchase Obligation, Due after Five Years
ALLETE Properties
Real Estate Investments, Net
Available-for-sale Securities
Available-for-sale Securities, Noncurrent
Other
Other Long-term Investments
Total Other Investments
Long-term Investments
Available-for-sale Corporate and Governmental Debt Securities, Maturities [Abstract]
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
One Year or Less
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value
One Year to Less Than Three Years
Available-for-sale Securities, Debt Maturities, After One Year Through Three Years, Fair Value
Amount of available-for-sale debt securities at fair value maturing after the first fiscal year through the third fiscal year following the latest fiscal year (maturing in more than one year to less than three years).
Three Years to Less Than Five Years
Available-for-sale Securities, Debt Maturities, After Three Years Through Five Years, Fair Value
Amount of available-for-sale debt securities at fair value maturing after the third fiscal year through the fifth fiscal year following the latest fiscal year (maturing in three years to less than five years).
Five or More Years
Available-for-sale Securities, Debt Maturities, After Five Years, Fair Value
Amount of available-for-sale debt securities at fair value maturing in after the fifth fiscal year following the latest fiscal year (maturing in five years or more).
Impairment of Land Inventory
Impairment of Real Estate
Supplemental Cash Flow Information [Abstract]
Cash Paid During the Period for Interest – Net of Amounts Capitalized
Interest Paid, Net
Noncash Investing and Financing Activities [Abstract]
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment
The decrease in accounts payable in the current period related to capital additions recognized in prior periods.
Capitalized Asset Retirement Costs
Increase (Decrease) in Asset Retirement Obligations
AFUDC–Equity
Increase (Decrease) in Allowance for Equity Funds Used During Construction
ALLETE Common Stock Contributed to the Pension Plan
Schedule of Business Acquisitions [Table Text Block]
Schedule of Business Acquisitions, by Acquisition [Table Text Block]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]
Recurring Fair Value Measures [Table Text Block]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]
Financial Instruments [Table Text Block]
Fair Value, by Balance Sheet Grouping [Table Text Block]
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Tax Expense [Abstract]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Federal
Current Federal Tax Expense (Benefit)
State
Current State and Local Tax Expense (Benefit)
Total Current Tax Expense
Current Income Tax Expense (Benefit)
Deferred Tax Expense [Abstract]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Federal
Deferred Federal Income Tax Expense (Benefit)
State
Deferred State and Local Income Tax Expense (Benefit)
Investment Tax Credit Amortization
Deferred Other Tax Expense (Benefit)
Total Deferred Tax Expense
Deferred Income Tax Expense (Benefit)
Total Income Tax Expense
Income Tax Expense (Benefit)
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Statutory Federal Income Tax Rate
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent
Income Taxes Computed at 35 percent Statutory Federal Rate
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount
Increase (Decrease) in Tax [Abstract]
Increase (Decrease) in Tax [Abstract]
Increase (Decrease) in Tax [Abstract]
State Income Taxes – Net of Federal Income Tax Benefit
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount
Production Tax Credits
Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount
Regulatory Differences for Utility Plant
Effective Income Tax Rate Reconciliation, Regulatory Differences
Regulatory Differences contributing to difference in effective tax rate to federal statutory rate.
Other
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount
Effective Tax Rate
Effective Income Tax Rate Reconciliation, Percent
Uncertain Tax Positions [Abstract]
Income Tax Uncertainties [Abstract]
Gross Unrecognized Tax Benefits
Unrecognized Tax Benefits
Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate
Unrecognized Tax Benefits that Would Impact Effective Tax Rate
Income Tax Expense [Table Text Block]
Schedule of Components of Income Tax Expense (Benefit) [Table Text Block]
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Table Text Block]
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]
Schedule of Short-Term and Long-Term Debt [Table Text Block]
Schedule of Debt [Table Text Block]
Regulatory Matters [Text Block]
Public Utilities Disclosure [Text Block]
Earnings Per Share [Abstract]
Reconciliation of Basic and Diluted Earnings Per Share [Table Text Block]
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]
Segment Reporting [Abstract]
Business Segments [Table]
Schedule of Segment Reporting Information, by Segment [Table]
Business Segments [Axis]
Segments [Axis]
Business Segments [Domain]
Segments [Domain]
Regulated Operations [Member]
Regulated Operations [Member] [Member]
Operation of entities that are regulated by governmental organizations for example, but not limited to, local, state, county, country, or other regulatory organizations.
U.S. Water Services [Member]
U.S. Water Services [Member]
U.S. Water Services [Member]
Corporate and Other [Member]
Corporate and Other [Member]
Business Segments [Line Items]
Segment Reporting Information [Line Items]
Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments
Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments
Number of Reportable Segments
Number of Reportable Segments
Number of Operating Segments
Number of Operating Segments
Land in Minnesota (Acres)
Area of Land
Operating Revenue
Revenues
Net Income (Loss) Attributable to ALLETE
Net Income (Loss) Attributable to Parent
Assets
Assets
Regulatory Matters [Table]
Public Utilities General Disclosures [Table]
Regulatory Agency [Axis]
Regulatory Agency [Axis]
Regulatory Agency [Domain]
Regulatory Agency [Domain]
MPUC [Member]
MPUC [Member]
Minnesota Public Utilities Commission [Member]
Transmission [Axis]
Transmission [Domain]
Regulatory Matters [Line Items]
Public Utilities, General Disclosures [Line Items]
Changes to Goodwill
Goodwill, Period Increase (Decrease)
Schedule of Finite-Lived Intangible Assets [Table]
Schedule of Finite-Lived Intangible Assets [Table]
Finite-Lived Intangible Assets by Major Class [Axis]
Finite-Lived Intangible Assets by Major Class [Axis]
Finite-Lived Intangible Assets, Major Class Name [Domain]
Finite-Lived Intangible Assets, Major Class Name [Domain]
Customer Relationships [Member]
Customer Relationships [Member]
Developed Technology and Other [Member]
Other Intangible Assets [Member]
Weighted Average [Member]
Weighted Average [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets [Line Items]
Definite-Lived Intangible Assets [Roll Forward]
Finite-lived Intangible Assets [Roll Forward]
Beginning Balance
Finite-Lived Intangible Assets, Net
Amortization
Amortization of Intangible Assets
Ending Balance
Remaining Useful Life (Years)
Finite-Lived Intangible Asset, Useful Life
Accumulated Amortization
Finite-Lived Intangible Assets, Accumulated Amortization
Estimated Annual Amortization Expense for Definite-Lived Intangible Assets [Abstract]
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Remainder of 2017
Finite-Lived Intangible Assets, Amortization Expense, Remainder of Fiscal Year
2018
Finite-Lived Intangible Assets, Amortization Expense, Year Two
2019
Finite-Lived Intangible Assets, Amortization Expense, Year Three
2020
Finite-Lived Intangible Assets, Amortization Expense, Year Four
2021
Finite-Lived Intangible Assets, Amortization Expense, Year Five
Thereafter
Finite-Lived Intangible Assets, Amortization Expense, after Year Five
Schedule of Indefinite-Lived Intangible Assets [Table]
Schedule of Indefinite-Lived Intangible Assets [Table]
Indefinite-lived Intangible Assets [Axis]
Indefinite-lived Intangible Assets [Axis]
Indefinite-lived Intangible Assets, Major Class Name [Domain]
Indefinite-lived Intangible Assets, Major Class Name [Domain]
Trademarks and Trade Names [Member]
Trademarks and Trade Names [Member]
Indefinite-lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets [Line Items]
Indefinite-Lived Intangible Assets [Roll Forward]
Indefinite-lived Intangible Assets [Roll Forward]
Beginning Balance
Indefinite-Lived Intangible Assets (Excluding Goodwill)
Ending Balance
Intangible Assets [Abstract]
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Total Intangible Assets
Intangible Assets, Net (Excluding Goodwill)
Total Intangible Assets, Amortization
Financial Covenants [Table]
Required Ratio of Indebtedness to Total Capitalization
Debt Instrument, Required Indebtedness to Total Capital Ratio
Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant.
Actual Ratio of Indebtedness to Total Capitalization
Debt Instrument, Actual Indebtedness to Total Capital Ratio
Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant.
Statement of Stockholders' Equity [Abstract]
Consolidated Statement of Shareholders' Equity [Table]
Statement [Table]
Equity Components [Axis]
Equity Components [Axis]
Total Shareholders' Equity [Domain]
Equity Component [Domain]
Retained Earnings [Member]
Retained Earnings [Member]
Accumulated Other Comprehensive Loss [Member]
AOCI Attributable to Parent [Member]
Common Stock [Member]
Common Stock [Member]
Consolidated Statement of Shareholders' Equity [Line Items]
Statement [Line Items]
Beginning Balance
Stockholders' Equity Attributable to Parent
Consolidated Statement of Shareholders' Equity [Roll Forward]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive Income [Abstract]
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract]
Net Income
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest
Other Comprehensive Income – Net of Tax [Abstract]
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Unrealized Gain on Securities
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax
Defined Benefit Pension and Other Postretirement Plans
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax
Total Comprehensive Income
Comprehensive Income (Loss), Net of Tax, Attributable to Parent
Common Stock Issued
Stock Issued During Period, Value, New Issues
Dividends Declared
Dividends, Common Stock
Ending Balance
Fair Value [Text Block]
Fair Value Disclosures [Text Block]
Other Investments [Table Text Block]
Investment [Table Text Block]
Rate Type [Axis]
Rate Type [Axis]
Rate Type [Axis] including categories like electric rates, natural gas rates, water rates, etc.
Rate Information by Type [Domain]
Rate Information by Type [Domain]
Rate Information by Type [Domain]
Electric Rates [Member]
Electric Rates [Member]
Electric Rates [Member]
Natural Gas Rates [Member]
Natural Gas Rates [Member]
Natural Gas Rates [Member]
Water Rates [Member]
Water Rates [Member]
Water Rates [Member]
FERC [Member]
FERC [Member]
Federal Energy Regulatory Commission [Member]
PSCW [Member]
PSCW [Member]
Public Service Corporation of Wisconsin [Member]
Regulatory Proceeding [Axis]
Public Utilities, Regulatory Proceeding [Axis]
Regulatory Proceeding [Domain]
Public Utilities, Regulatory Proceeding [Domain]
2010 Minnesota Rate Case [Member]
2010 Rate Case [Member]
Retail rate increase request filed with the Minnesota Public Utilities Commission (MPUC).
Minnesota Cost Recovery Riders [Member]
Current Cost Recovery Rider [Member]
Current Cost Recovery Rider [Member]
2016 Minnesota General Rate Case [Member]
2016 Minnesota Rate Case [Member] [Member]
2016 Minnesota Rate Case [Member] [Member]
FERC-Approved Wholesale Rates [Member]
FERC-Approved Wholesale Rates [Member]
Wholesale rates approved by the Federal Energy Regulatory Commission (FERC).
Renewable Cost Recovery Rider [Member]
Renewable Cost Recovery Rider [Member]
Renewable Cost Recovery Rider [Member]
2012 Wisconsin General Rate Case [Member]
2012 Wisconsin Rate Case [Member]
2012 retail rate increase requested filed with the Public Service Commission of Wisconsin (PSCW).
2016 Wisconsin General Rate Case [Member]
2016 Wisconsin Rate Case [Member]
2016 retail rate increase request filed with the PSCW [Member]
SWL&P [Member]
SWL&P [Member]
Superior, Water, Light and Power [Member]
Customer Class [Axis]
Customer Class [Axis]
Customer Class [Axis] description of customer types or classes
Customer Class [Domain]
Customer Class [Domain]
[Domain] for Customer Class [Axis] description of customer types or classes
Retail Customers [Member]
Retail Customers [Member]
Retail Customers [Member] include residential customers, commercial customers and industrial customers.
Municipal Customers [Member]
Municipal Customers [Member]
Municipal Customers [Member] include cities, governments and other municipalities
Subsequent Event Type [Axis]
Subsequent Event Type [Axis]
Subsequent Event Type [Domain]
Subsequent Event Type [Domain]
Subsequent Event [Member]
Subsequent Event [Member]
Long-term Contracts for Sale of Electric Power [Axis]
Long-term Contracts for Sale of Electric Power [Axis]
Long-term Contracts for Sale of Electric Power [Axis]
Long-term Contracts for Sale of Electric Power [Domain]
Long-term Contracts for Sale of Electric Power [Domain]
[Domain] for Long-term Contracts for Sale of Electric Power [Axis]
Wholesale Electric Contracts (Expire December 2024) [Member]
Wholesale Electric Contracts (Expire December 2024) [Member]
Wholesale Electric Contracts (Expire December 2024) [Member]
Wholesale Electric Contract (Termination Effective June 2019) [Member]
Wholesale Electric Contract (Termination Effective June 2019) [Member]
Wholesale Electric Contract (Termination Effective June 2019) [Member]
Wholesale Electric Contract (Cost-Based Formula Methodology for Entire Term) [Member]
Wholesale Electric Contract (Cost-Based Formula Methodology for Entire Term) [Member]
Wholesale Electric Contracts (Cost-Based Formula Methodology for Entire Term) [Member]
Generating Facility [Axis]
Generating Facility Axis [Axis]
Name of generating unit or facility [Axis]
Generating Facility Unit Name [Domain]
Generating Facility Unit Name [Domain]
[Domain] for Name of generating unit or facility [Axis]
Boswell Energy Center [Member]
Boswell Energy Center [Member]
Boswell Energy Center [Member]
Approved Return on Common Equity
Public Utilities, Approved Return on Equity, Percentage
Approved Equity Ratio
Public Utilities, Approved Equity Capital Structure, Percentage
Revenue from Cost Recovery Riders
Regulated Operating Revenue, Other
Requested Rate Increase (Decrease)
Public Utilities, Requested Rate Increase (Decrease), Percentage
Requested Return on Equity
Public Utilities, Requested Return on Equity, Percentage
Requested Equity Ratio
Public Utilities, Requested Equity Capital Structure, Percentage
Annual Additional Revenue Generated from Requested Final Rate Increase
Public Utilities, Requested Rate Increase (Decrease), Amount
Annual Additional Revenue Generated from Requested Interim Rate Increase
Public Utilities, Interim Rate Increase (Decrease), Amount
Expected Decrease in Annual Depreciation Expense
Expected increase (Decrease) in Annual Depreciation Expense Based on a Regulatory Order
Expected increase (decrease) in annual depreciation expense based on a regulatory order, if approved.
Number of Customers
Number of Customers
Number of Customers
Notice Required to Terminate (Years)
Length of Notice Required to Terminate (Years)
The length of notice required to terminate a contract (in years).
Average Monthly Demand (MW)
Average Customer Demand (MW)
Average demand measured for a specified customer, measured in MW.
Change in Annual Capacity Charge Per Contract, Percentage
Change in Annual Capacity Charge Per Contract, Percentage
The annual percentage change of the capacity charge specified in a power purchase, power sales, or electric sales agreement.
Reduction in Operating Revenue due to Regulatory Outcome
Regulated Operating Revenue
After-Tax Charge for North Dakota Investment Tax Credits Reversed
Tax Adjustments, Settlements, and Unusual Provisions
Acquisitions [Text Block]
Business Combination Disclosure [Text Block]
Guarantor Obligations [Table]
Schedule of Guarantor Obligations [Table]
Guarantee Entity and Type [Axis]
Guarantee Entity and Type [Axis]
Guarantee Entity and Type [Axis]
Guarantee Entity and Type [Domain]
Guarantee Entity and Type [Domain]
[Domain] for Guarantee Entity and Type [Axis]
BNI Energy Reclamation Liability [Member]
BNI Coal Reclamation Liability [Member]
Reclamation liability for closing costs associated with BNI Coal mine and mine facilities.
ALLETE Properties Development and Maintenance Obligations [Member]
ALLETE Properties Performance and Maintenance Obligations [Member]
Performance and maintenance obligations to governmental entities to construct improvements in various projects.
Guarantor Obligations, Nature [Axis]
Guarantor Obligations, Nature [Axis]
Guarantor Obligations, Nature [Domain]
Guarantor Obligations, Nature [Domain]
Letters of Credit [Member]
Financial Standby Letter of Credit [Member]
Surety Bonds [Member]
Surety Bond [Member]
Surety Bonds and Letters of Credit [Member]
Surety Bonds and Letters of Credit [Member]
Surety Bonds and Letters of Credit [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations [Line Items]
Collateral
Guarantor Obligations, Collateral Held Directly or by Third Parties, Amount
Amount of any assets held either as collateral or by third parties that, upon the occurence of any triggering event or condition under the guarantee, can be obtained and liquidated to recover all or a portion of the amounts paid under the guarantee.
Estimated Obligation
Guarantor Obligations, Maximum Exposure, Undiscounted
ALLETE's Investment in ATC [Table Text Block]
Equity Method Investments [Table Text Block]
Short-Term and Long-Term Debt [Text Block]
Debt Disclosure [Text Block]
Statement of Comprehensive Income [Abstract]
Comprehensive Income [Abstract]
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net Income
Other Comprehensive Income (Loss) [Abstract]
Unrealized Gain (Loss) on Securities Net of Income Tax Expense (Benefit) of $0.3 and $(0.3)
Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $0.1 and $0.1
Total Other Comprehensive Income (Loss)
Other Comprehensive Income (Loss), Net of Tax
Total Comprehensive Income
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest
Less: Non-Controlling Interest in Subsidiaries
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest
Total Comprehensive Income Attributable to ALLETE
Components of Net Periodic Benefit Expense (Income) [Table Text Block]
Schedule of Net Benefit Costs [Table Text Block]
Assets [Abstract]
Current Assets [Abstract]
Assets, Current [Abstract]
Cash and Cash Equivalents
Cash and Cash Equivalents, at Carrying Value
Accounts Receivable (Less Allowance of $2.4 and $3.1)
Accounts Receivable, Net, Current
Inventories – Net
Inventory, Net
Prepayments and Other
Other Assets, Current
Total Current Assets
Assets, Current
Property, Plant and Equipment – Net
Property, Plant and Equipment, Net
Regulatory Assets
Regulatory Assets, Noncurrent
Investment in ATC
Equity Method Investments
Other Investments
Goodwill and Intangible Assets – Net
Intangible Assets, Net (Including Goodwill)
Other Non-Current Assets
Other Assets, Noncurrent
Total Assets
Liabilities and Shareholders' Equity [Abstract]
Liabilities and Equity [Abstract]
Liabilities [Abstract]
Current Liabilities [Abstract]
Liabilities, Current [Abstract]
Accounts Payable
Accounts Payable, Current
Accrued Taxes
Taxes Payable, Current
Accrued Interest
Interest Payable, Current
Long-Term Debt Due Within One Year
Long-term Debt, Current Maturities
Notes Payable
Notes Payable, Current
Other
Other Liabilities, Current
Total Current Liabilities
Liabilities, Current
Long-Term Debt
Deferred Income Taxes
Deferred Tax Liabilities, Net, Noncurrent
Regulatory Liabilities
Regulatory Liability, Noncurrent
Defined Benefit Pension and Other Postretirement Benefit Plans
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent
Other Non-Current Liabilities
Other Liabilities, Noncurrent
Total Liabilities
Liabilities
Commitments, Guarantees and Contingencies (Note 13)
Commitments and Contingencies
Shareholders’ Equity [Abstract]
Stockholders' Equity Attributable to Parent [Abstract]
Common Stock Without Par Value, 80.0 Shares Authorized, 50.9 and 49.6 Shares Issued and Outstanding
Common Stock, Value, Issued
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax
Retained Earnings
Retained Earnings (Accumulated Deficit)
Total Shareholders’ Equity
Total Liabilities and Shareholders' Equity
Liabilities and Equity
Income Statement [Abstract]
Operating Expenses [Abstract]
Costs and Expenses [Abstract]
Fuel and Purchased Power
Cost of Purchased Power
Transmission Services
Cost of Transmission
Cost of Sales
Cost of Goods and Services Sold
Operating and Maintenance
Operating Expenses
Depreciation and Amortization
Depreciation, Depletion and Amortization
Taxes Other than Income Taxes
Taxes, Other
Total Operating Expenses
Costs and Expenses
Operating Income
Operating Income (Loss)
Other Income (Expense) [Abstract]
Nonoperating Income (Expense) [Abstract]
Interest Expense
Interest Expense
Equity Earnings in ATC
Income (Loss) from Equity Method Investments
Other
Other Nonoperating Income (Expense)
Total Other Expense
Nonoperating Income (Expense)
Income Tax Expense
Net Income
Less: Non-Controlling Interest in Subsidiaries
Net Income (Loss) Attributable to Noncontrolling Interest
Net Income Attributable to ALLETE
Average Shares of Common Stock and Per Share Data [Abstract]
Basic (Shares)
Weighted Average Number of Shares Outstanding, Basic
Diluted (Shares)
Weighted Average Number of Shares Outstanding, Diluted
Basic Earnings Per Share of Common Stock
Earnings Per Share, Basic
Diluted Earnings Per Share of Common Stock
Earnings Per Share, Diluted
Dividends Per Share of Common Stock
Common Stock, Dividends, Per Share, Declared
Business Segments [Text Block]
Segment Reporting Disclosure [Text Block]
Reason for Business Acquisitions
Business Combination, Reason for Business Combination
Earnings Per Share and Common Stock [Text Block]
Earnings Per Share [Text Block]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Table]
Measurement Frequency [Axis]
Fair Value, Measurement Frequency [Domain]
Nonrecurring Fair Value Measurements [Member]
Fair Value, Measurements, Nonrecurring [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Abstract]
Indicators of Impairment
Asset Impairment Charges
Fair Value of Financial Instruments [Table]
Fair Value, by Balance Sheet Grouping [Table]
Fair Value, Hierarchy [Axis]
Fair Value Hierarchy [Domain]
Fair Value of Financial Instruments [Line Items]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-Term Debt, Including Long-Term Debt Due Within One Year - Carrying Amount
Long-term Debt
Long-Term Debt, Including Long-Term Debt Due Within One Year - Fair Value
Long-term Debt, Fair Value
Power Purchase Agreements [Table]
Long-term Contracts for Purchase of Electric Power [Table]
Square Butte [Member]
Square Butte [Member]
Square Butte [Member]
Minnkota Power [Member]
Minnkota Power [Member]
Minnkota Power [Member]
Square Butte PPA [Member]
Long-term contract with Square Butte for the purchase of electric power.
Square Butte Coal-fired Unit [Member]
Square Butte Coal-fired Unit [Member]
Square Butte Coal-fired Unit located near Center, North Dakota
Power Sales Agreements [Axis]
Power Sales Agreements [Domain]
Minnkota Power PSA [Member]
Minnkota Sales Agreement [Member]
Minnkota Sales Agreement [Member]
Power Purchase Agreements [Line Items]
Long-term Contract for Purchase of Electric Power [Line Items]
Generating Capacity (MW)
Capacity of a generating facility, unit, or project in megawatts.
Output Entitlement
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased
PPA Counterparty Total Debt Outstanding
Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding
Cost of Power Purchased
Utilities Operating Expense, Purchased Power under Long-term Contracts
Pro Rata Share of PPA Counterparty Interest Expense
Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges
Minnesota Solar Energy Standard [Member]
Minnesota Solar Energy Standard [Member]
Minnesota Solar Energy Standard [Member]
Minnesota Solar Energy Standard - Camp Ripley Project [Member]
Minnesota Solar Energy Standard - Camp Ripley Project [Member]
Minnesota Solar Energy Standard - Camp Ripley Project [Member]
Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member]
Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member]
Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member]
Minnesota Solar Energy Standard - Community Solar Garden Project - Owned and Operated [Member]
Minnesota Solar Energy Standard - Community Solar Garden Project - Owned and Operated [Member]
Minnesota Solar Energy Standard - Community Solar Garden Project - Owned and Operated [Member]
Solar Energy Standard Mandate - Overall Mandate Percentage
Solar Energy Standard Mandate, Overall Mandate Percentage
Minnesota's solar energy mandate for total retail electric sales, excluding sales to certain industrial customers, by the end of 2020.
Solar Energy Standard Mandate - Small Scale Solar Mandate Percentage
Solar Energy Standard Mandate, Small Scale Solar Mandate Percentage
Minnesota's solar energy mandate for small scale solar with photovoltaic devices with a nameplate capacity of 20 kilowatts or less by the end of 2020. Expressed as a percentage of the overall mandate.
Solar Energy Standard Mandate - Qualifying Capacity for Small Scale Solar Mandate (MW)
Solar Energy Standard Mandate, Qualifying Capacity for Small Scale Solar Mandate
The maximum amount of nameplate capacity of a solar device to qualify for counting towards the small scale solar mandate of Minnesota's solar energy standard.
Number of Projects
Number of Projects
Number of projects.
Solar Energy Standard Mandate - Percentage of Overall Mandate Expected to be Met with Current Filings or Projects
Solar Energy Standard Mandate, Percentage of Overall Mandate Expected to be Met with Current Filings or Projects
The percentage of the solar energy standard overall mandate expected to be met with current filings and projects.
Unrealized Gain (Loss) on Securities, Income Tax Expense (Benefit)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax
Defined Benefit Pension and Other Postretirement Benefit Plans, Income Tax Expense
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax
Operations and Significant Accounting Policies [Text Block]
Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block]
Schedule of Stock by Class [Table]
Schedule of Stock by Class [Table]
Class of Stock [Axis]
Class of Stock [Axis]
Class of Stock [Domain]
Class of Stock [Domain]
Class of Stock [Line Items]
Class of Stock [Line Items]
Options to Purchase Shares Excluded from Computation of Diluted Earnings Per Share
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount
Employer Contributions to Defined Benefit Pension Plan - Common Stock Shares
Stock Issued During Period, Shares, Employee Benefit Plan
Reconciliation of Basic and Diluted Earnings Per Share [Abstract]
Earnings Per Share Reconciliation [Abstract]
Earnings Per Share - Basic [Abstract]
Earnings Per Share, Basic [Abstract]
Net Income Attributable to ALLETE
Net Income (Loss) Available to Common Stockholders, Basic
Average Common Shares
Earnings Per Share
Earnings Per Share - Diluted [Abstract]
Earnings Per Share, Diluted [Abstract]
Net Income Attributable to ALLETE
Net Income (Loss) Available to Common Stockholders, Diluted
Average Common Shares
Earnings Per Share
Dilutive Securities (Shares)
Weighted Average Number Diluted Shares Outstanding Adjustment
Environmental Matters [Table]
Environmental Matters [Table]
Environmental Matters [Table]
Environmental Remediation Contingency [Axis]
Environmental Remediation Contingency [Axis]
Environmental Remediation Contingency [Domain]
Environmental Remediation Contingency [Domain]
NOV Consent Decree [Member]
NOV Consent Decree [Member]
NOV Consent Decree [Member]
Ozone NAAQS [Member]
Ozone NAAQS [Member]
Ozone NAAQS [Member]
SO2 and NO2 NAAQS [Member]
SO2 and NO2 NAAQS [Member]
SO2 and NO2 NAAQS [Member]
Clean Water Act - Aquatic Organisms [Member]
Clean Water Act - Aquatic Organisms [Member]
Clean Water Act - Aquatic Organisms [Member]
Coal Combustion Residuals [Member]
Coal Combustion Residuals [Member]
Coal Combustion Residuals [Member]
Environmental Protection Agency Violations [Axis]
Environmental Protection Agency Violations [Axis]
Environmental Protection Agency Violations [Axis]
Environmental Protection Agency Violations [Domain]
Environmental Protection Agency Violations [Domain]
[Domain] for Environmental Protection Agency Violations [Axis]
Federal Insecticide, Fungicide and Rodenticle Act [Member]
Federal Insecticide, Fungicide and Rodenticle Act [Member]
Federal Insecticide, Fungicide and Rodenticle Act [Member]
Environmental Matters [Line Items]
Environmental Matters [Line Items]
[Line Items] for Environmental Matters [Table]
Additional Wind Generating Capacity (MW)
Estimated Costs of Compliance
Site Contingency, Loss Exposure Not Accrued, Best Estimate
Estimated Costs of Compliance
Site contingency, Loss Exposure Accrued and Best Estimate of Loss Exposure in Excess of Accrual
Amount of loss exposure accrued and the best estimate of reasonably possible loss exposure in excess of the accrual.
Settlement, Amount
Settlement, Amount
Amount of judgment or settlement awarded to or against the entity.
Investment in ATC [Table]
Schedule of Equity Method Investments [Table]
Equity Method Investment [Axis]
Investment, Name [Axis]
Equity Method Investment [Domain]
Investment, Name [Domain]
ATC [Member]
ATC [Member]
ATC [Member]
Return on Equity Complaint 2 [Member]
FERC Complaint 2 [Member]
FERC Complaint 2 [Member]
Sensitivity Analysis [Axis]
Sensitivity Analysis [Axis]
The hypothetical effect on future earnings using different variables of assumptions.
Sensitivity Analysis [Domain]
Sensitivity Analysis [Domain]
[Domain] for The hypothetical effect on future earnings using different variables of assumptions.
Sensitivity Analysis [Member]
Sensitivity Analysis [Member]
[Member] for The hypothetical effect on future earnings using different variables of assumptions.
Expense Presentation [Axis]
Expense Presentation [Axis]
Expense Presentation [Axis]
Expense Presentation [Domain]
Expense Presentation [Domain]
[Domain] for Expense Presentation [Axis]
After-tax [Member]
After-tax [Member]
After-tax [Member]
Investment in ATC [Line Items]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage
Equity Method Investment, Ownership Percentage
Expected Additional Investment in 2017
Equity Method Investment, Expected Additional Investment
Additional investments expected to be made during the current fiscal year but after the filing date, not already separately disclosed as part of current period/year-to-date investments or as a subsequent event.
ALLETE's Investment in ATC [Roll Forward]
ALLETE's Investment in ATC [Roll Forward]
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2016
Cash Investments
Payments to Acquire Equity Method Investments
Equity in ATC Earnings
Distributed ATC Earnings
Proceeds from Equity Method Investment, Dividends or Distributions
Equity Investment Balance as of March 31, 2017
Authorized Return on Equity
Equity Method Investment, Approved Return on Common Equity
The approved percentage return on common equity dedicated to the utility plant in the investee accounted for under the equity method of accounting.
Authorized Return on Equity, Including Incentive Adder
Equity Method Investment, Approved Return on Common Equity, Including Incentive Adder
The approved percentage return on common equity, including an incentive adder for participation in a regional transmission organization dedicated to the utility plant in the investee accounted for under the equity method of accounting.
Proposed Return on Equity
Equity Method Investment, Proposed Return on Common Equity
The proposed return on common equity, which would be dedicated to the utility plant in the investee accounted for under the equity method of accounting.
Proposed Return on Equity, Including Incentive Adder
Equity Method Investment, Proposed Return on Common Equity, Including Incentive Adder
The proposed return on common equity, including an incentive adder for participation in a regional transmission organization, which would be dedicated to the utility plant in the investee accounted for under the equity method of accounting.
Basis Point Reduction on Approved Rate of Return on Common Equity
Basis Point Reduction on Approved Rate of Return on Common Equity
A sensitivity analysis using basis point reduction.
Annual Effect on Future Equity Earnings in ATC
Effect on Future Earnings, Amount
Regulatory Assets and Liabilities Disclosure [Abstract]
Schedule of Regulatory Assets and Liabilities [Table]
Schedule of Regulatory Assets and Liabilities [Table]
Regulatory Asset [Axis]
Total Regulatory Asset [Domain]
Regulatory Asset [Domain]
Deferred Fuel Adjustment Clause [Member]
Deferred Fuel Costs [Member]
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
Pension and Other Postretirement Plans Costs [Member]
Income Taxes [Member]
Deferred Income Tax Charge [Member]
Asset Retirement Obligations [Member]
Asset Retirement Obligation Costs [Member]
Cost Recovery Riders [Member]
PPACA Income Tax Deferral [Member]
Medicare Part D Regulatory Asset [Member]
Discloses the amount of regulatory asset arising from the deferral of the retail portion of a tax charge taken in 2010 resulting from the Patient Protection and Affordable Care Act of 2010 (PPACA).
Other
Other Regulatory Assets (Liabilities) [Member]
Regulatory Liability [Axis]
Regulatory Liability [Axis]
Regulatory Liability [Domain]
Regulatory Liability [Domain]
Wholesale and Retail Contra AFUDC [Member]
Wholesale and Retail Contra AFUDC [Member]
Wholesale and Retail Contra AFUDC [Member]
North Dakota Investment Tax Credits [Member]
North Dakota Investment Tax Credits Subject to Refund [Member]
North Dakota Investment Tax Credits Subject to Refund [Member]
Plant Removal Obligations [Member]
Removal Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets and Liabilities Currently Earning a Return
Types of Net Regulatory Assets Earning Returns
Current Regulatory Assets
Regulatory Assets, Current
Non-Current Regulatory Assets
Total Regulatory Assets
Regulatory Assets
Non-Current Regulatory Liabilities
Integrated Resource Plan [Member]
Integrated Resource Plan [Member]
Integrated Resource Plan which contains a utility's analysis of existing and future energy resource alternatives and a projection of customer cost impact by class [Member]
Generation Type [Axis]
Generation Type [Axis]
Generation Type [Axis]
Generation Type [Domain]
Generation Type [Domain]
[Domain] for Generation Type [Axis]
Natural Gas-Fired [Member]
Natural Gas Processing Plant [Member]
Leasing Agreements [Table]
Schedule of Operating Leased Assets [Table]
Leasing Agreements [Axis]
Property, Plant and Equipment, Type [Axis]
Leasing Agreements [Domain]
Property, Plant and Equipment, Type [Domain]
BNI Energy Dragline [Member]
Exploration and Production Equipment [Member]
Leasing Agreements [Line Items]
Operating Leased Assets [Line Items]
Minimum Lease Payments Due Annually
Operating Leases, Future Minimum Payments Due, Next Twelve Months
Termination Fee
Termination Fee
A lump sum payment in conjunction with a lease termination.
Minimum Lease Payments Due for the Remainder of 2017
Operating Leases, Future Minimum Payments, Remainder of Fiscal Year
Minimum Lease Payments Due in 2018
Operating Leases, Future Minimum Payments, Due in Two Years
Minimum Lease Payments Due in 2019
Operating Leases, Future Minimum Payments, Due in Three Years
Minimum Lease Payments Due in 2020
Operating Leases, Future Minimum Payments, Due in Four Years
Minimum Lease Payments Due in 2021
Operating Leases, Future Minimum Payments, Due in Five Years
Minimum Lease Payments Thereafter
Operating Leases, Future Minimum Payments, Due Thereafter
Schedule of Intangible Assets [Table Text Block]
Schedule of Intangible Assets and Goodwill [Table Text Block]
Loss Contingencies [Table]
Loss Contingencies [Table]
Return on Equity Complaint 1 [Member]
FERC Complaint 1 [Member]
FERC Complaint 1 [Member]
Loss Contingencies [Line Items]
Loss Contingencies [Line Items]
Requested Return on Equity Filed with the FERC by Third Party
Requested Return on Common Equity Filed with Regulatory Organization
The requested return on common equity, which would be dedicated to the utilities within the regulatory organizations jurisdiction.
FERC Authorized Return on Common Equity
FERC Authorized Return on Common Equity
FERC Authorized Return on Common Equity
FERC Authorized Return on Equity Including Incentive Adder
FERC Authorized Return on Equity, Including Incentive Adder
FERC Authorized Return on Equity, Including Incentive Adder
Proposed Return on Equity by Federal Administrative Law Judge
Proposed Return on Common Equity by Regulatory Organization
The proposed return on common equity by regulatory organization
Proposed Return on Equity by Federal Administrative Law Judge Including Incentive Adder
Proposed Return on Equity by Regulatory Organization, Including Incentive Adder
Proposed Return on Equity by Regulatory Organization, Including Incentive Adder
Statement of Cash Flows [Abstract]
Operating Activities [Abstract]
Net Cash Provided by (Used in) Operating Activities [Abstract]
AFUDC – Equity
Income from Equity Investments – Net of Dividends
Income (Loss) from Equity Method Investments, Net of Dividends or Distributions
Change in Fair Value of Contingent Consideration
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability
Loss on Sales of Property, Plant and Equipment
Gain (Loss) on Disposition of Assets
Depreciation Expense
Depreciation
Amortization of PSAs
Amortization of Power Purchase Agreements
The amount of income recognized in the current period that reflects income associated with the reduced liability of the PPAs acquired in a business acquisition. As noncash income, this element is subtracted from net income when calculating cash provided by or used in operations using the indirect method.
Amortization of Other Intangible Assets and Other Assets
Other Depreciation and Amortization
Deferred Income Tax Expense
Share-Based Compensation Expense
Share-based Compensation
ESOP Compensation Expense
Other Noncash Expense
Defined Benefit Pension and Postretirement Benefit Expense
Pension and Other Postretirement Benefit Expense
Bad Debt Expense (Recoveries)
Provision for Doubtful Accounts
Changes in Operating Assets and Liabilities [Abstract]
Increase (Decrease) in Operating Capital [Abstract]
Accounts Receivable
Increase (Decrease) in Accounts and Other Receivables
Inventories
Increase (Decrease) in Inventories
Prepayments and Other
Increase (Decrease) in Prepaid Expense and Other Assets
Accounts Payable
Increase (Decrease) in Accounts Payable
Other Current Liabilities
Increase (Decrease) in Other Current Liabilities
Cash Contributions to Defined Benefit Pension Plans
Changes in Regulatory and Other Non-Current Assets
Increase (Decrease) in Other Noncurrent Assets
Changes in Regulatory and Other Non-Current Liabilities
Increase (Decrease) in Other Noncurrent Liabilities
Cash from Operating Activities
Net Cash Provided by (Used in) Operating Activities
Investing Activities [Abstract]
Net Cash Provided by (Used in) Investing Activities [Abstract]
Proceeds from Sale of Available-for-sale Securities
Proceeds from Sale of Available-for-sale Securities
Payments for Purchase of Available-for-sale Securities
Payments to Acquire Available-for-sale Securities
Investment in ATC
Changes to Other Investments
Payments for (Proceeds from) Other Investing Activities
Additions to Property, Plant and Equipment
Payments to Acquire Property, Plant, and Equipment
Proceeds from Sale of Property, Plant and Equipment
Proceeds from Sale of Productive Assets
Cash for Investing Activities
Net Cash Provided by (Used in) Investing Activities
Financing Activities [Abstract]
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from Issuance of Common Stock
Changes in Restricted Cash
Proceeds from (Repayments of) Restricted Cash, Financing Activities
Changes in Notes Payable
Proceeds from (Repayments of) Notes Payable
Repayments of Long-Term Debt
Repayments of Long-term Debt
Acquisition-Related Contingent Consideration Payments
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements
Dividends on Common Stock
Payments of Ordinary Dividends, Common Stock
Cash for Financing Activities
Net Cash Provided by (Used in) Financing Activities
Change in Cash and Cash Equivalents
Cash and Cash Equivalents, Period Increase (Decrease)
Cash and Cash Equivalents at Beginning of Period
Cash and Cash Equivalents at End of Period
Business Segments [Table Text Block]
Schedule of Segment Reporting Information, by Segment [Table Text Block]
Pension and Other Postretirement Benefit Plans [Text Block]
Pension and Other Postretirement Benefits Disclosure [Text Block]
Investments [Text Block]
Investment [Text Block]
Inventories – Net [Abstract]
Inventory, Net [Abstract]
Fuel
Energy Related Inventory, Coal
Materials and Supplies
Inventory, Supplies, Net of Reserves
Raw Materials
Inventory, Raw Materials, Gross
Work in Progress
Inventory, Work in Process, Gross
Finished Goods
Inventory, Finished Goods, Gross
Reserve for Obsolescence
Inventory Valuation Reserves
Total Inventories – Net
Prepayments and Other Current Assets [Abstract]
Prepaid Expense and Other Assets, Current [Abstract]
Deferred Fuel Adjustment Clause
Deferred Fuel Cost
Restricted Cash
Restricted Cash and Cash Equivalents, Current
Other
Other Assets, Miscellaneous, Current
Total Prepayments and Other Current Assets
Other Non-Current Assets [Abstract]
Other Assets, Noncurrent [Abstract]
Contract Payment
Contract Payment Non-Current
Carrying value of a non-current deferred asset related to a payment made as part of a long-term contract.
Finance Receivable
Financing Receivable, Gross
Restricted Cash
Restricted Cash and Cash Equivalents, Noncurrent
Other
Other Assets, Miscellaneous, Noncurrent
Total Other Non-Current Assets
Other Current Liabilites [Abstract]
Other Liabilities, Current [Abstract]
PSAs
Power Purchase Agreement - Current Liability
The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a current liability.
Contingent Consideration
Other
Other Accrued Liabilities, Current
Total Other Current Liabilities
Other Non-Current Liabilities [Abstract]
Other Liabilities, Noncurrent [Abstract]
Asset Retirement Obligation
Asset Retirement Obligations, Noncurrent
PSAs
Power Purchase Agreement - Noncurrent Liability
The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a noncurrent liability.
Contingent Consideration
Business Combination, Contingent Consideration, Liability, Noncurrent
Other
Other Accrued Liabilities, Noncurrent
Total Other Non-Current Liabilities