1.0.0.3 false Statement Of Income Alternative (USD $) In Thousands, except Per Share data false 1 $ false false Shares Standard http://www.xbrl.org/2003/instance shares 0 USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 USDperShareItemType Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares 0 false 2 $ false false Shares Standard http://www.xbrl.org/2003/instance shares 0 USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 USDperShareItemType Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares 0 false 3 $ false false Shares Standard http://www.xbrl.org/2003/instance shares 0 USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 USDperShareItemType Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares 0 false 4 $ false false Shares Standard http://www.xbrl.org/2003/instance shares 0 USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 USDperShareItemType Divide http://www.xbrl.org/2003/iso4217 USD iso4217 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 814743000 814743 false false 2 true true 953500000 953500 false false 3 true true 1771916000 1771916 false false 4 true true 1828526000 1828526 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 4 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 216766000 216766 false false 2 false true 245252000 245252 false false 3 false true 475727000 475727 false false 4 false true 495078000 495078 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 112227000 112227 false false 2 false true 97207000 97207 false false 3 false true 246122000 246122 false false 4 false true 194587000 194587 false false No definition available. No authoritative reference available. false 9 Deferred energy costs, net 4 aye_DeferredEnergyCostsNet aye false debit duration monetary No definition available. false false false false false false false false false 1 false true -7574000 -7574 false false 2 false true 1103000 1103 false false 3 false true -24582000 -24582 false false 4 false true -9351000 -9351 false false No definition available. 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 200519000 200519 false false 2 false true 189931000 189931 false false 3 false true 367694000 367694 false false 4 false true 358631000 358631 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 67221000 67221 false false 2 false true 68794000 68794 false false 3 false true 135732000 135732 false false 4 false true 139083000 139083 false false No definition available. No authoritative reference available. false 12 Taxes other than income taxes 4 us-gaap_TaxesOtherThanIncomeExciseProductionAndPropertyTaxes us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 46509000 46509 false false 2 false true 52879000 52879 false false 3 false true 102323000 102323 false false 4 false true 105318000 105318 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 635668000 635668 false false 2 false true 655166000 655166 false false 3 false true 1303016000 1303016 false false 4 false true 1283346000 1283346 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 179075000 179075 false false 2 false true 298334000 298334 false false 3 false true 468900000 468900 false false 4 false true 545180000 545180 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 1758000 1758 false false 2 false true 4661000 4661 false false 3 false true 4217000 4217 false false 4 false true 10870000 10870 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 59070000 59070 false false 2 false true 58704000 58704 false false 3 false true 116350000 116350 false false 4 false true 117135000 117135 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 121763000 121763 false false 2 false true 244291000 244291 false false 3 false true 356767000 356767 false false 4 false true 438915000 438915 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 48900000 48900 false false 2 false true 89820000 89820 false false 3 false true 149806000 149806 false false 4 false true 148113000 148113 false false No definition available. No authoritative reference available. false 19 Net income 3 aye_ProfitLoss aye false credit duration monetary No definition available. false false false false false false false false false 1 false true 72863000 72863 false false 2 false true 154471000 154471 false false 3 false true 206961000 206961 false false 4 false true 290802000 290802 false false No definition available. No authoritative reference available. true 20 Less net income attributable to noncontrolling interest 3 aye_NetIncomeLossAttributableToNoncontrollingInterest aye false debit duration monetary No definition available. false false false false false false false false false 1 false true -255000 -255 false false 2 false true -345000 -345 false false 3 false true -428000 -428 false false 4 false true -551000 -551 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 72608000 72608 false false 2 true true 154126000 154126 false false 3 true true 206533000 206533 false false 4 true true 290251000 290251 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 4 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 0.43 0.43 false false 2 true true 0.92 0.92 false false 3 true true 1.22 1.22 false false 4 true true 1.73 1.73 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 0.43 0.43 false false 2 true true 0.91 0.91 false false 3 true true 1.22 1.22 false false 4 true true 1.71 1.71 false false No definition available. No authoritative reference available. false 25 Average shares outstanding: 3 us-gaap_SharesUsedInComputationOfEarningsLossPerShareAbstract 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 4 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 169506000 169506 false false 2 false true 168236000 168236 false false 3 false true 169473000 169473 false false 4 false true 167898000 167898 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 169908000 169908 false false 2 false true 170106000 170106 false false 3 false true 169888000 169888 false false 4 false true 170028000 170028 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 0.15 0.15 false false 2 true true 0.15 0.15 false false 3 true true 0.30 0.30 false false 4 true true 0.30 0.30 false false No definition available. No authoritative reference available. false false 4 24 false Thousands Thousands Hundreds false true