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Operations and Significant Accounting Policies - Reclassifications (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended | 3 Months Ended
--- | --- | ---
Dec. 31, 2014 | Mar. 31, 2014
Consolidated Balance Sheet [Member] | Property, Plant and Equipment - Net to Goodwill and Intangible Assets - Net [Member] |  |
Reclassifications [Line Items] |  |
Reclassification Adjustment | $ 1.6 us-gaap_PriorPeriodReclassificationAdjustment / us-gaap_BalanceSheetLocationAxis = us-gaap_PropertyPlantAndEquipmentMember / ale_ConsolidatedFinancialStatementAxis = ale_ConsolidatedBalanceSheetMember |
Consolidated Balance Sheet [Member] | Other Non-Current Assets to Goodwill and Intangible Assets - Net [Member] |  |
Reclassifications [Line Items] |  |
Reclassification Adjustment | 3.2 us-gaap_PriorPeriodReclassificationAdjustment / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherNoncurrentAssetsMember / ale_ConsolidatedFinancialStatementAxis = ale_ConsolidatedBalanceSheetMember |
Consolidated Balance Sheet [Member] | Total Assets [Member] |  |
Reclassifications [Line Items] |  |
Reclassification Adjustment | 0 us-gaap_PriorPeriodReclassificationAdjustment / us-gaap_BalanceSheetLocationAxis = us-gaap_AssetsTotalMember / ale_ConsolidatedFinancialStatementAxis = ale_ConsolidatedBalanceSheetMember |
Consolidated Statement of Income [Member] | Operating and Maintenance Expenses to Cost of Sales [Member] |  |
Reclassifications [Line Items] |  |
Reclassification Adjustment |  | 23.5 us-gaap_PriorPeriodReclassificationAdjustment / ale_ConsolidatedFinancialStatementAxis = ale_ConsolidatedStatementofIncomeMember / us-gaap_IncomeStatementLocationAxis = us-gaap_OperatingExpenseMember
Consolidated Statement of Income [Member] | Operating and Maintenance Expenses to Transmission Services [Member] |  |
Reclassifications [Line Items] |  |
Reclassification Adjustment |  | 10.8 us-gaap_PriorPeriodReclassificationAdjustment / ale_ConsolidatedFinancialStatementAxis = ale_ConsolidatedStatementofIncomeMember / us-gaap_IncomeStatementLocationAxis = ale_OperatingExpenseTransmissionServicesMember
Consolidated Statement of Income [Member] | Operating and Maintenance Expenses to Taxes Other than Income Taxes [Member] |  |
Reclassifications [Line Items] |  |
Reclassification Adjustment |  | 11.2 us-gaap_PriorPeriodReclassificationAdjustment / ale_ConsolidatedFinancialStatementAxis = ale_ConsolidatedStatementofIncomeMember / us-gaap_IncomeStatementLocationAxis = ale_OperatingExpenseTaxesOtherthanIncomeTaxesMember
Consolidated Statement of Income [Member] | Operating Income [Member] |  |
Reclassifications [Line Items] |  |
Reclassification Adjustment |  | 0 us-gaap_PriorPeriodReclassificationAdjustment / ale_ConsolidatedFinancialStatementAxis = ale_ConsolidatedStatementofIncomeMember / us-gaap_IncomeStatementLocationAxis = us-gaap_OperatingIncomeLossMember
Consolidated Statement of Income [Member] | Net Income [Member] |  |
Reclassifications [Line Items] |  |
Reclassification Adjustment |  | 0 us-gaap_PriorPeriodReclassificationAdjustment / ale_ConsolidatedFinancialStatementAxis = ale_ConsolidatedStatementofIncomeMember / us-gaap_IncomeStatementLocationAxis = ale_NetIncomeBeforeNonControllingInterestinSubsidiariesMember
Consolidated Statement of Income [Member] | Net Income Attributable to ALLETE [Member] |  |
Reclassifications [Line Items] |  |
Reclassification Adjustment |  | 0 us-gaap_PriorPeriodReclassificationAdjustment / ale_ConsolidatedFinancialStatementAxis = ale_ConsolidatedStatementofIncomeMember / us-gaap_IncomeStatementLocationAxis = ale_NetIncomeAttributabletoParentMember
Consolidated Statement of Cash Flows [Member] | Changes in Regulatory and Other Non-Current Liabilities to Amortization of Power Purchase Agreements [Member] |  |
Reclassifications [Line Items] |  |
Reclassification Adjustment |  | 2.3 us-gaap_PriorPeriodReclassificationAdjustment / ale_CashFlowStatementLocationAxis = ale_ChangesinRegulatoryandOtherNonCurrentLiabilitiesMember / ale_ConsolidatedFinancialStatementAxis = ale_ConsolidatedStatementofCashFlowsMember
Consolidated Statement of Cash Flows [Member] | Operating Activities [Member] |  |
Reclassifications [Line Items] |  |
Reclassification Adjustment |  | 0 us-gaap_PriorPeriodReclassificationAdjustment / ale_CashFlowStatementLocationAxis = ale_OperatingActivitiesMember / ale_ConsolidatedFinancialStatementAxis = ale_ConsolidatedStatementofCashFlowsMember
Consolidated Statement of Cash Flows [Member] | Investing Activities [Member] |  |
Reclassifications [Line Items] |  |
Reclassification Adjustment |  | 0 us-gaap_PriorPeriodReclassificationAdjustment / ale_CashFlowStatementLocationAxis = ale_InvestingActivitiesMember / ale_ConsolidatedFinancialStatementAxis = ale_ConsolidatedStatementofCashFlowsMember
Consolidated Statement of Cash Flows [Member] | Financing Activities [Member] |  |
Reclassifications [Line Items] |  |
Reclassification Adjustment |  | $ 0 us-gaap_PriorPeriodReclassificationAdjustment / ale_CashFlowStatementLocationAxis = ale_FinancingActivitiesMember / ale_ConsolidatedFinancialStatementAxis = ale_ConsolidatedStatementofCashFlowsMember

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