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Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning Accumulated Other Comprehensive Loss | $ (21.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax |  | $ (17.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax |
Other Comprehensive Income Before Reclassifications | 0.3 us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax |  | 0 us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax |
Amounts Reclassified From Accumulated Other Comprehensive Loss | 0.2 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax |  | 0.3 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax |
Net Other Comprehensive Income | 0.5 us-gaap_OtherComprehensiveIncomeLossNetOfTax |  | 0.3 us-gaap_OtherComprehensiveIncomeLossNetOfTax |
Ending Accumulated Other Comprehensive Loss | (20.6)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax |  | (16.8)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax |
Reclassifications Out of Accumulated Other Comprehensive Income [Abstract] |  |  |  |
AOCI Reclassification, Unrealized Gains on Available-for-sale Securities, Before Tax | 0 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax |  | 0 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax |
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Before Tax | (0.5)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansBeforeTax |  | (0.5)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansBeforeTax |
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 | [1] | 0 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax / us-gaap_IncomeStatementLocationAxis = ale_IncomeTaxExpenseBenefitMember | [1]
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Tax | 0.2 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansTax / us-gaap_IncomeStatementLocationAxis = ale_IncomeTaxExpenseBenefitMember | [1] | 0.2 us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansTax / us-gaap_IncomeStatementLocationAxis = ale_IncomeTaxExpenseBenefitMember | [1]
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, Prior Service Costs, Before Tax | 0.1 us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax / us-gaap_IncomeStatementLocationAxis = us-gaap_OperatingExpenseMember | [2] | 0.1 us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax / us-gaap_IncomeStatementLocationAxis = us-gaap_OperatingExpenseMember | [2]
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Actuarial Gains and Losses, Before Tax | (0.6)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_IncomeStatementLocationAxis = us-gaap_OperatingExpenseMember | [2] | (0.6)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax / us-gaap_IncomeStatementLocationAxis = us-gaap_OperatingExpenseMember | [2]
Unrealized Gains and Losses on Available-for-sale Securities [Member] |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning 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 |
Other Comprehensive Income Before Reclassifications | 0.2 us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |  | 0 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 |  | 0 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |
Net Other Comprehensive Income | 0.1 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |  | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |
Ending Accumulated Other Comprehensive Loss | (0.2)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |  | (0.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |
Defined Benefit Pension and Other Postretirement Items [Member] |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning 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 |
Other Comprehensive Income Before Reclassifications | 0 us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  | 0 us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |
Amounts Reclassified From Accumulated Other Comprehensive Loss | 0.3 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  | 0.3 us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |
Net Other Comprehensive Income | 0.3 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  | 0.3 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |
Ending Accumulated Other Comprehensive Loss | (20.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |  | (16.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |
Gains and Losses on Cash Flow Hedge [Member] |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning 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 |
Other Comprehensive Income Before Reclassifications | 0.1 us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |  | 0 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 | 0.1 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |  | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |
Ending Accumulated Other Comprehensive Loss | $ 0 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |  | $ (0.3)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |
[1] | Included in Income Tax Expense on our Consolidated Statement of Income.
[2] | 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 15. Pension and Other Postretirement Benefit Plans.)

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- Definition Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge. + 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 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e681-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669686-108580 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 31 -Article 5 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 + Details
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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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