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Common Stock and Earnings Per Share - Summary of Common Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2014 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Class of Stock [Line Items] |  |  |  |  |
Common Stock Outstanding, Shares |  | 45,900,000 us-gaap_CommonStockSharesOutstanding | 41,400,000 us-gaap_CommonStockSharesOutstanding |  |
Contributions to Pension, Value |  | $ 19.5 us-gaap_StockIssuedDuringPeriodValueEmployeeBenefitPlan | $ 0 us-gaap_StockIssuedDuringPeriodValueEmployeeBenefitPlan | $ 0 us-gaap_StockIssuedDuringPeriodValueEmployeeBenefitPlan |
Common Stock [Member] |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Common Stock Outstanding, Shares |  | 45,929,000 us-gaap_CommonStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 41,401,000 us-gaap_CommonStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 39,377,000 us-gaap_CommonStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 37,513,000 us-gaap_CommonStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember
Common Stock Outstanding, Value |  | 1,107.6 us-gaap_CommonStockValueOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 885.2 us-gaap_CommonStockValueOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 784.7 us-gaap_CommonStockValueOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 705.6 us-gaap_CommonStockValueOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember
Employee Stock Purchase Program, Shares |  | 18,000 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 16,000 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 20,000 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |
Employee Stock Purchase Program, Value |  | 0.8 us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 0.7 us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 0.8 us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |
Invest Direct, Shares |  | 378,000 us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 395,000 us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 474,000 us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |
Invest Direct, Value |  | 18.9 us-gaap_StockIssuedDuringPeriodValueDividendReinvestmentPlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 18.5 us-gaap_StockIssuedDuringPeriodValueDividendReinvestmentPlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 19.2 us-gaap_StockIssuedDuringPeriodValueDividendReinvestmentPlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |
Options and Stock Awards, Shares |  | 78,000 ale_StockIssuedDuringPeriodSharesShareBasedCompensationAndEsopGross / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 301,000 ale_StockIssuedDuringPeriodSharesShareBasedCompensationAndEsopGross / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 95,000 ale_StockIssuedDuringPeriodSharesShareBasedCompensationAndEsopGross / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |
Options and Stock Awards, Value |  | 8.0 ale_StockIssuedDuringPeriodValueShareBasedCompensationAndEsopGross / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 17.9 ale_StockIssuedDuringPeriodValueShareBasedCompensationAndEsopGross / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 6.0 ale_StockIssuedDuringPeriodValueShareBasedCompensationAndEsopGross / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |
Equity Issuance Program, Shares |  | 1,851,000 us-gaap_StockIssuedDuringPeriodSharesOther / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 1,312,000 us-gaap_StockIssuedDuringPeriodSharesOther / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 1,275,000 us-gaap_StockIssuedDuringPeriodSharesOther / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |
Equity Issuance Program, Value |  | 90.0 us-gaap_StockIssuedDuringPeriodValueOther / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 63.4 us-gaap_StockIssuedDuringPeriodValueOther / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 53.1 us-gaap_StockIssuedDuringPeriodValueOther / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |
Forward Sale Agreement and Issuance, Shares |  | 1,807,000 ale_StockIssuedDuringPeriodSharesForwardSale / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |
Forward Sales Agreement and Issuance, Value |  | 85.2 ale_StockIssuedDuringPeriodValueForwardSale / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |
Contributions to Pension, Shares | 400,000 us-gaap_StockIssuedDuringPeriodSharesEmployeeBenefitPlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 396,000 us-gaap_StockIssuedDuringPeriodSharesEmployeeBenefitPlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 0 us-gaap_StockIssuedDuringPeriodSharesEmployeeBenefitPlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 0 us-gaap_StockIssuedDuringPeriodSharesEmployeeBenefitPlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |
Contributions to Pension, Value | $ 19.5 us-gaap_StockIssuedDuringPeriodValueEmployeeBenefitPlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | $ 19.5 us-gaap_StockIssuedDuringPeriodValueEmployeeBenefitPlan / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |
Equity Issuance Program Shares Authorized |  | 9,600,000 ale_EquityIssuanceProgramSharesAuthorized / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |
Equity Issuance Program Shares Available for Issuance |  | 1,300,000 ale_EquityIssuanceProgramSharesAvailableForIssuance / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |
Antidilutive Options to Purchase Shares of Common Stock Excluded from the Computation of Earnings Per Share |  |  | 0 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | 200,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |

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