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Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |  |
Beginning Accumulated Other Comprehensive Loss | $ (17.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax |  | $ (22.0)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax |  |
Other Comprehensive Income (Loss) Before Reclassifications | (5.3)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax |  | 4.6 us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax |  |
Amounts Reclassified from Accumulated Other Comprehensive Loss | 1.3 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax |  | 0.3 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax |  |
Net Other Comprehensive Income (Loss) | (4.0)us-gaap_OtherComprehensiveIncomeLossNetOfTax |  | 4.9 us-gaap_OtherComprehensiveIncomeLossNetOfTax |  | 6.9 us-gaap_OtherComprehensiveIncomeLossNetOfTax
Ending Accumulated Other Comprehensive Loss | (21.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax |  | (17.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax |  | (22.0)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Other Income (Expense) - Other [Member] |  |  |  |  |
Reclassifications Out of Accumulated Other Comprehensive Income [Abstract] |  |  |  |  |
AOCI Reclassification, Unrealized Gains on Available-for-sale Securities, Before Tax | (0.2)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax / us-gaap_IncomeStatementLocationAxis = us-gaap_OtherIncomeMember | [1] | 2.2 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax / us-gaap_IncomeStatementLocationAxis = us-gaap_OtherIncomeMember | [1] |
Income Tax Expense [Member] |  |  |  |  |
Reclassifications Out of Accumulated Other Comprehensive Income [Abstract] |  |  |  |  |
AOCI Reclassification, Unrealized Gains on Available-for-sale Securities, Tax | 0.1 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax / us-gaap_IncomeStatementLocationAxis = ale_IncomeTaxExpenseBenefitMember | [2] | (0.9)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax / us-gaap_IncomeStatementLocationAxis = ale_IncomeTaxExpenseBenefitMember | [2] |
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Tax | 0.8 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansTax / us-gaap_IncomeStatementLocationAxis = ale_IncomeTaxExpenseBenefitMember | [2] | 1.1 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansTax / us-gaap_IncomeStatementLocationAxis = ale_IncomeTaxExpenseBenefitMember | [2] |
Operating Expenses - Operating and Maintenance [Member] |  |  |  |  |
Reclassifications Out of Accumulated Other Comprehensive Income [Abstract] |  |  |  |  |
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Before Tax | (2.0)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansBeforeTax / us-gaap_IncomeStatementLocationAxis = us-gaap_OperatingExpenseMember |  | (2.7)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansBeforeTax / us-gaap_IncomeStatementLocationAxis = us-gaap_OperatingExpenseMember |  |
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement items, Prior Service Costs, Before Tax | 0.3 us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax / us-gaap_IncomeStatementLocationAxis = us-gaap_OperatingExpenseMember | [3] | 0.8 us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax / us-gaap_IncomeStatementLocationAxis = us-gaap_OperatingExpenseMember | [3] |
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Actuarial Gains and Losses, Before Tax | (2.3)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_IncomeStatementLocationAxis = us-gaap_OperatingExpenseMember | [3] | (3.5)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_IncomeStatementLocationAxis = us-gaap_OperatingExpenseMember | [3] |
Unrealized Gains and Losses on Available-for-sale Securities [Member] |  |  |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |  |
Beginning Accumulated Other Comprehensive Loss | (0.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |  | (0.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |  |
Other Comprehensive Income (Loss) Before Reclassifications | (0.3)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |  | 1.3 us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |  |
Amounts Reclassified from Accumulated Other Comprehensive Loss | 0.1 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |  | (1.3)us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |  |
Net Other Comprehensive Income (Loss) | (0.2)us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |  | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |  |
Ending Accumulated Other Comprehensive Loss | (0.3)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |  | (0.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |  |
Defined Benefit Pension, Other Postretirement Items [Member] |  |  |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |  |
Beginning Accumulated Other Comprehensive Loss | (16.7)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  | (21.5)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  |
Other Comprehensive Income (Loss) Before Reclassifications | (5.2)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  | 3.2 us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  |
Amounts Reclassified from Accumulated Other Comprehensive Loss | 1.2 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  | 1.6 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  |
Net Other Comprehensive Income (Loss) | (4.0)us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  | 4.8 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  |
Ending Accumulated Other Comprehensive Loss | (20.7)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  | (16.7)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  |
Gains and Losses on Cash Flow Hedge [Member] |  |  |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |  |
Beginning Accumulated Other Comprehensive Loss | (0.3)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |  | (0.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |  |
Other Comprehensive Income (Loss) Before Reclassifications | 0.2 us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |  | 0.1 us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |  |
Amounts Reclassified from Accumulated Other Comprehensive Loss | 0 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |  | 0 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |  |
Net Other Comprehensive Income (Loss) | 0.2 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |  | 0.1 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |  |
Ending Accumulated Other Comprehensive Loss | $ (0.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |  | $ (0.3)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |  |
[1] | Included in Other Income (Expense) – Other on our Consolidated Statement of Income.
[2] | Included in Income Tax Expense on our Consolidated Statement of Income.
[3] | Defined benefit pension and other postretirement items excluded from our Regulated Operations are recognized in accumulated other comprehensive loss and are subsequently reclassified out of accumulated other comprehensive loss as components of net periodic pension and other postretirement benefit expense. (See Note 17. Pension and Other Postretirement Benefit Plans.)

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- Definition Amount before tax of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) related to pension and other postretirement benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 + Details
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