1.0.0.3 false Statement Of Income Alternative (USD $) In Millions, except Per Share data false 1 $ false false iso4217_USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 iso4217_USD_per_shares Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares 0 shares Standard http://www.xbrl.org/2003/instance shares 0 false 2 $ false false iso4217_USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 iso4217_USD_per_shares Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares 0 shares Standard http://www.xbrl.org/2003/instance shares 0 false 3 $ false false iso4217_USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 iso4217_USD_per_shares Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares 0 shares Standard http://www.xbrl.org/2003/instance shares 0 5 Operating revenues 3 us-gaap_Revenues us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 true true 3426800000 3426.8 false false 2 true true 3385900000 3385.9 false false 3 true true 3307000000 3307.0 false false No definition available. No authoritative reference available. false 6 Operating expenses: 3 us-gaap_CostsAndExpensesAbstract us-gaap true na duration string No definition available. false false false false false true false false false 1 false false 0 0 false false 2 false false 0 0 false false 3 false false 0 0 false false No definition available. false 7 Fuel 4 us-gaap_FuelCosts us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 886600000 886.6 false false 2 false true 1080900000 1080.9 false false 3 false true 930800000 930.8 false false No definition available. No authoritative reference available. false 8 Purchased power and transmission 4 us-gaap_CostOfPurchasedPower us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 502000000 502.0 false false 2 false true 395600000 395.6 false false 3 false true 393200000 393.2 false false No definition available. No authoritative reference available. false 9 Deferred energy costs, net 4 aye_DeferredEnergyCostsNet aye false debit duration monetary Represents the deferral of certain costs incurred to the period in which such costs are recovered in rates. false false false false false false false false false 1 false true -64400000 -64.4 false false 2 false true -63700000 -63.7 false false 3 false true -10100000 -10.1 false false Represents the deferral of certain costs incurred to the period in which such costs are recovered in rates. No authoritative reference available. false 10 Operations and maintenance 4 us-gaap_UtilitiesOperatingExpenseMaintenanceAndOperations us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 687100000 687.1 false false 2 false true 674800000 674.8 false false 3 false true 687000000 687.0 false false No definition available. No authoritative reference available. false 11 Depreciation and amortization 4 us-gaap_DepreciationAndAmortization us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 282100000 282.1 false false 2 false true 273900000 273.9 false false 3 false true 277000000 277.0 false false No definition available. No authoritative reference available. false 12 Taxes other than income taxes 4 us-gaap_TaxesExcludingIncomeAndExciseTaxes us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 213600000 213.6 false false 2 false true 214900000 214.9 false false 3 false true 211800000 211.8 false false No definition available. No authoritative reference available. false 13 Total operating expenses 4 us-gaap_CostsAndExpenses us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 2507000000 2507.0 false false 2 false true 2576400000 2576.4 false false 3 false true 2489700000 2489.7 false false No definition available. No authoritative reference available. true 14 Operating income 3 us-gaap_OperatingIncomeLoss us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 919800000 919.8 false false 2 false true 809500000 809.5 false false 3 false true 817300000 817.3 false false No definition available. No authoritative reference available. true 15 Other income (expense), net 3 us-gaap_OtherNonoperatingIncomeExpense us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 7000000 7.0 false false 2 false true 22300000 22.3 false false 3 false true 36800000 36.8 false false No definition available. No authoritative reference available. false 16 Interest expense 3 us-gaap_InterestExpense us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 291100000 291.1 false false 2 false true 231900000 231.9 false false 3 false true 187300000 187.3 false false No definition available. No authoritative reference available. false 17 Income before income taxes 3 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 635700000 635.7 false false 2 false true 599900000 599.9 false false 3 false true 666800000 666.8 false false No definition available. No authoritative reference available. true 18 Income tax expense 3 us-gaap_IncomeTaxExpenseBenefit us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 241600000 241.6 false false 2 false true 204100000 204.1 false false 3 false true 250800000 250.8 false false No definition available. No authoritative reference available. false 19 Net income 3 us-gaap_ProfitLoss us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 394100000 394.1 false false 2 false true 395800000 395.8 false false 3 false true 416000000 416.0 false false No definition available. No authoritative reference available. true 20 Net income attributable to noncontrolling interests 3 us-gaap_NetIncomeLossAttributableToNoncontrollingInterest us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true -1300000 -1.3 false false 2 false true -400000 -0.4 false false 3 false true -3800000 -3.8 false false No definition available. No authoritative reference available. false 21 Net income attributable to Allegheny Energy, Inc. 3 us-gaap_NetIncomeLoss us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 true true 392800000 392.8 false false 2 true true 395400000 395.4 false false 3 true true 412200000 412.2 false false No definition available. No authoritative reference available. true 22 Earnings per share attributable to Allegheny Energy, Inc.: 3 us-gaap_EarningsPerShareAbstract us-gaap true na duration string No definition available. false false false false false true false false false 1 false false 0 0 false false 2 false false 0 0 false false 3 false false 0 0 false false No definition available. false 23 Basic 4 us-gaap_EarningsPerShareBasic us-gaap true na duration decimal No definition available. false false false false false false false false true 1 true true 2.32 2.32 false false 2 true true 2.35 2.35 false false 3 true true 2.48 2.48 false false No definition available. No authoritative reference available. false 24 Diluted 4 us-gaap_EarningsPerShareDiluted us-gaap true na duration decimal No definition available. false false false false false false false false true 1 true true 2.31 2.31 false false 2 true true 2.33 2.33 false false 3 true true 2.43 2.43 false false No definition available. No authoritative reference available. false 25 Average shares outstanding: 3 us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract us-gaap true na duration string No definition available. false false false false false true false false false 1 false false 0 0 false false 2 false false 0 0 false false 3 false false 0 0 false false No definition available. false 26 Basic 4 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic us-gaap true na duration shares No definition available. false false false false false false false false false 1 false true 169500000 169.5 false false 2 false true 168500000 168.5 false false 3 false true 166000000 166.0 false false No definition available. No authoritative reference available. false 27 Diluted 4 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding us-gaap true na duration shares No definition available. false false false false false false false false false 1 false true 170000000 170.0 false false 2 false true 170000000 170.0 false false 3 false true 169500000 169.5 false false No definition available. No authoritative reference available. false 28 Dividends per share 3 us-gaap_CommonStockDividendsPerShareCashPaid us-gaap true na duration decimal No definition available. false false false false false false false false true 1 true true 0.60 0.60 false false 2 true true 0.60 0.60 false false 3 true true 0.15 0.15 false false No definition available. No authoritative reference available. false false 3 24 false HundredThousands HundredThousands Hundreds false true