1.0.0.3 false Statement Of Financial Position Classified (USD $) In Millions 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 6 Current Assets: 4 us-gaap_AssetsCurrentAbstract 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 No definition available. false 7 Cash and cash equivalents 5 us-gaap_CashAndCashEquivalentsAtCarryingValue us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 true true 286600000 286.6 false false 2 true true 362100000 362.1 false false No definition available. No authoritative reference available. false 8 Accounts receivable: 5 us-gaap_AccountsReceivableNetCurrentAbstract 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 No definition available. false 9 Customer 6 us-gaap_AccountsReceivableGrossCurrent us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 188200000 188.2 false false 2 false true 188300000 188.3 false false No definition available. No authoritative reference available. false 10 Unbilled utility revenue 6 us-gaap_ReceivablesLongTermContractsOrPrograms us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 116400000 116.4 false false 2 false true 122700000 122.7 false false No definition available. No authoritative reference available. false 11 Wholesale and other 6 us-gaap_OtherReceivables us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 64400000 64.4 false false 2 false true 61500000 61.5 false false No definition available. No authoritative reference available. false 12 Allowance for uncollectible accounts 6 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true -14000000 -14.0 false false 2 false true -13300000 -13.3 false false No definition available. No authoritative reference available. false 13 Materials and supplies 5 us-gaap_InventoryNet us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 110600000 110.6 false false 2 false true 115100000 115.1 false false No definition available. No authoritative reference available. false 14 Fuel 5 us-gaap_EnergyRelatedInventory us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 206400000 206.4 false false 2 false true 128200000 128.2 false false No definition available. No authoritative reference available. false 15 Deferred income taxes 5 us-gaap_DeferredTaxAssetsNetCurrent us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 81500000 81.5 false false 2 false true 69600000 69.6 false false No definition available. No authoritative reference available. false 16 Prepaid taxes 5 us-gaap_PrepaidTaxes us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 48400000 48.4 false false 2 false true 44800000 44.8 false false No definition available. No authoritative reference available. false 17 Collateral deposits 5 us-gaap_DepositAssets us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 20800000 20.8 false false 2 false true 33400000 33.4 false false No definition available. No authoritative reference available. false 18 Derivative assets 5 us-gaap_DerivativeAssetsCurrent us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 4600000 4.6 false false 2 false true 113100000 113.1 false false No definition available. No authoritative reference available. false 19 Restricted funds 5 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 25900000 25.9 false false 2 false true 36800000 36.8 false false No definition available. No authoritative reference available. false 20 Regulatory assets 5 us-gaap_RegulatoryAssetsCurrent us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 132700000 132.7 false false 2 false true 158800000 158.8 false false No definition available. No authoritative reference available. false 21 Assets held for sale 5 us-gaap_AssetsHeldForSaleCurrent us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 32400000 32.4 false false 2 false true 0 0 false false No definition available. No authoritative reference available. false 22 Other 5 us-gaap_OtherAssetsCurrent us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 40400000 40.4 false false 2 false true 74600000 74.6 false false No definition available. No authoritative reference available. false 23 Total current assets 5 us-gaap_AssetsCurrent us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 1345300000 1345.3 false false 2 false true 1495700000 1495.7 false false No definition available. No authoritative reference available. true 24 Property, Plant and Equipment: 4 us-gaap_PropertyPlantAndEquipmentNetAbstract 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 No definition available. false 25 Generation 5 aye_PropertyPlantAndEquipmentGenerationOrProcessing aye false debit instant monetary Period end amount of property, plant and equipment (PPE) related to generation or processing. false false false false false false false false false 1 false true 7469400000 7469.4 false false 2 false true 6107300000 6107.3 false false Period end amount of property, plant and equipment (PPE) related to generation or processing. No authoritative reference available. false 26 Transmission 5 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmission us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 1313200000 1313.2 false false 2 false true 1179500000 1179.5 false false No definition available. No authoritative reference available. false 27 Distribution 5 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDistribution us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 3784400000 3784.4 false false 2 false true 3944700000 3944.7 false false No definition available. No authoritative reference available. false 28 Other 5 us-gaap_PropertyPlantAndEquipmentOther us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 440700000 440.7 false false 2 false true 455000000 455.0 false false No definition available. No authoritative reference available. false 29 Accumulated depreciation 5 us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true -5104900000 -5104.9 false false 2 false true -4994100000 -4994.1 false false No definition available. No authoritative reference available. false 30 Subtotal 5 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 7902800000 7902.8 false false 2 false true 6692400000 6692.4 false false No definition available. No authoritative reference available. true 31 Construction work in progress 5 us-gaap_ConstructionInProgressGross us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 800600000 800.6 false false 2 false true 1309800000 1309.8 false false No definition available. No authoritative reference available. false 32 Property, plant and equipment held for sale, net 5 us-gaap_AssetsHeldForSalePropertyPlantAndEquipment us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 253700000 253.7 false false 2 false true 0 0 false false No definition available. No authoritative reference available. false 33 Total property, plant and equipment, net 5 us-gaap_PropertyPlantAndEquipmentNet us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 8957100000 8957.1 false false 2 false true 8002200000 8002.2 false false No definition available. No authoritative reference available. true 34 Other Noncurrent Assets: 4 us-gaap_OtherAssetsNoncurrentAbstract 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 No definition available. false 35 Regulatory assets 5 us-gaap_RegulatoryAssetsNoncurrent us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 717300000 717.3 false false 2 false true 687700000 687.7 false false No definition available. No authoritative reference available. false 36 Goodwill 5 us-gaap_Goodwill us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 367300000 367.3 false false 2 false true 367300000 367.3 false false No definition available. No authoritative reference available. false 37 Restricted funds 5 us-gaap_RestrictedCashAndCashEquivalentsNoncurrent us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 60200000 60.2 false false 2 false true 133300000 133.3 false false No definition available. No authoritative reference available. false 38 Investments in unconsolidated affiliates 5 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 26700000 26.7 false false 2 false true 28000000 28.0 false false No definition available. No authoritative reference available. false 39 Derivative assets 5 us-gaap_DerivativeAssetsNoncurrent us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 0 0 false false 2 false true 9800000 9.8 false false No definition available. No authoritative reference available. false 40 Other 5 us-gaap_OtherAssetsNoncurrent us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 115200000 115.2 false false 2 false true 87000000 87.0 false false No definition available. No authoritative reference available. false 41 Total other noncurrent assets 5 aye_TotalOtherAssets aye false debit instant monetary Total Other Assets false false false false false false false false false 1 false true 1286700000 1286.7 false false 2 false true 1313100000 1313.1 false false Total Other Assets No authoritative reference available. true 42 Total Assets 4 us-gaap_Assets us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true 11589100000 11589.1 false false 2 false true 10811000000 10811.0 false false No definition available. No authoritative reference available. true 44 Current Liabilities: 4 us-gaap_LiabilitiesCurrentAbstract 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 No definition available. false 45 Long-term debt due within one year 5 us-gaap_LongTermDebtCurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 140800000 140.8 false false 2 false true 93900000 93.9 false false No definition available. No authoritative reference available. false 46 Accounts payable 5 us-gaap_AccountsPayableCurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 411400000 411.4 false false 2 false true 368000000 368.0 false false No definition available. No authoritative reference available. false 47 Accrued taxes 5 us-gaap_TaxesPayableCurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 87300000 87.3 false false 2 false true 119400000 119.4 false false No definition available. No authoritative reference available. false 48 Payable to PJM for FTRs 5 us-gaap_AccountsPayableOtherCurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 31700000 31.7 false false 2 false true 110800000 110.8 false false No definition available. No authoritative reference available. false 49 Derivative liabilities 5 us-gaap_DerivativeLiabilitiesCurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 24400000 24.4 false false 2 false true 22200000 22.2 false false No definition available. No authoritative reference available. false 50 Regulatory liabilities 5 us-gaap_RegulatoryLiabilityCurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 37400000 37.4 false false 2 false true 69200000 69.2 false false No definition available. No authoritative reference available. false 51 Accrued interest 5 us-gaap_InterestPayableCurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 68300000 68.3 false false 2 false true 58100000 58.1 false false No definition available. No authoritative reference available. false 52 Security deposits 5 us-gaap_CustomerDepositsCurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 51000000 51.0 false false 2 false true 46200000 46.2 false false No definition available. No authoritative reference available. false 53 Liabilities associated with assets held for sale 5 aye_LiabilitiesOfAssetsHeldForSaleCurrent aye false credit instant monetary Liability (such as a mortgage) related to a disposal group that is held for sale and anticipated to be sold within one year... false false false false false false false false false 1 false true 10100000 10.1 false false 2 false true 0 0 false false Liability (such as a mortgage) related to a disposal group that is held for sale and anticipated to be sold within one year or the normal operating cycle. The liability is expected to be discharged as part of the plan of sale for the asset. No authoritative reference available. false 54 Other 5 us-gaap_OtherLiabilitiesCurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 123200000 123.2 false false 2 false true 115800000 115.8 false false No definition available. No authoritative reference available. false 55 Total current liabilities 5 us-gaap_LiabilitiesCurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 985600000 985.6 false false 2 false true 1003600000 1003.6 false false No definition available. No authoritative reference available. true 56 Long-term Debt 4 us-gaap_LongTermDebtNoncurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 4417000000 4417.0 false false 2 false true 4115900000 4115.9 false false No definition available. No authoritative reference available. false 57 Deferred Credits and Other Liabilities: 4 us-gaap_OtherLiabilitiesNoncurrentAbstract 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 No definition available. false 58 Derivative liabilities 5 us-gaap_DerivativeLiabilitiesNoncurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 6700000 6.7 false false 2 false true 11900000 11.9 false false No definition available. No authoritative reference available. false 59 Income taxes payable 5 us-gaap_AccruedIncomeTaxesNoncurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 85700000 85.7 false false 2 false true 75700000 75.7 false false No definition available. No authoritative reference available. false 60 Investment tax credit 5 us-gaap_AccumulatedDeferredInvestmentTaxCredit us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 61600000 61.6 false false 2 false true 65800000 65.8 false false No definition available. No authoritative reference available. false 61 Deferred income taxes 5 us-gaap_DeferredTaxLiabilitiesNoncurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 1501300000 1501.3 false false 2 false true 1277400000 1277.4 false false No definition available. No authoritative reference available. false 62 Regulatory liabilities 5 us-gaap_RegulatoryLiabilityNoncurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 461200000 461.2 false false 2 false true 528900000 528.9 false false No definition available. No authoritative reference available. false 63 Pension and other postretirement employee benefit plan liabilities 5 us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 597400000 597.4 false false 2 false true 578400000 578.4 false false No definition available. No authoritative reference available. false 64 Adverse power purchase commitment 5 us-gaap_LiabilityAboveMarketForNonutilityGeneratorContractsNoncurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 114400000 114.4 false false 2 false true 132300000 132.3 false false No definition available. No authoritative reference available. false 65 Liabilities associated with assets held for sale 5 aye_LiabilitiesOfAssetsHeldForSaleNoncurrent aye false credit instant monetary Long lived liabilities that are held for sale. false false false false false false false false false 1 false true 53100000 53.1 false false 2 false true 0 0 false false Long lived liabilities that are held for sale. No authoritative reference available. false 66 Other 5 us-gaap_OtherLiabilitiesNoncurrent us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 177000000 177.0 false false 2 false true 165400000 165.4 false false No definition available. No authoritative reference available. false 67 Total deferred credits and other liabilities 5 aye_TotalDeferredCreditsAndOtherLiabilities aye false credit instant monetary Total deferred credits and other liabilities. false false false false false false false false false 1 false true 3058400000 3058.4 false false 2 false true 2835800000 2835.8 false false Total deferred credits and other liabilities. No authoritative reference available. true 69 Equity: 4 us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract 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 No definition available. false 70 Common stock-$1.25 par value per share, 260 million shares authorized and 169,620,917 and 169,413,887 shares issued at December 31, 2009 and 2008, respectively 5 us-gaap_CommonStockValue us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 212000000 212.0 false false 2 false true 211800000 211.8 false false No definition available. No authoritative reference available. false 71 Other paid-in capital 5 us-gaap_AdditionalPaidInCapitalCommonStock us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 1970200000 1970.2 false false 2 false true 1952500000 1952.5 false false No definition available. No authoritative reference available. false 72 Retained earnings 5 us-gaap_RetainedEarningsAccumulatedDeficit us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 1022700000 1022.7 false false 2 false true 731600000 731.6 false false No definition available. No authoritative reference available. false 73 Treasury stock at cost- 51,313 and 49,493 shares at December 31, 2009 and 2008, respectively 5 us-gaap_TreasuryStockValue us-gaap true debit instant monetary No definition available. false false false false false false false false false 1 false true -1800000 -1.8 false false 2 false true -1800000 -1.8 false false No definition available. No authoritative reference available. false 74 Accumulated other comprehensive loss 5 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true -89900000 -89.9 false false 2 false true -43300000 -43.3 false false No definition available. No authoritative reference available. false 75 Total Allegheny Energy, Inc. common stockholders' equity 5 us-gaap_StockholdersEquity us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 3113200000 3113.2 false false 2 false true 2850800000 2850.8 false false No definition available. No authoritative reference available. true 76 Noncontrolling interest 5 us-gaap_MinorityInterest us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 14900000 14.9 false false 2 false true 4900000 4.9 false false No definition available. No authoritative reference available. false 77 Total equity 5 us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 false true 3128100000 3128.1 false false 2 false true 2855700000 2855.7 false false No definition available. No authoritative reference available. true 78 Total Liabilities and Equity 4 us-gaap_LiabilitiesAndStockholdersEquity us-gaap true credit instant monetary No definition available. false false false false false false false false false 1 true true 11589100000 11589.1 false false 2 true true 10811000000 10811.0 false false No definition available. No authoritative reference available. true false 2 71 false HundredThousands UnKnown UnKnown false true