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Pension and Other Postretirement Benefits - Schedule of Amounts Included in AOCL and Regulatory Assets (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Pension Plans, Postretirement and Other Employee Benefits [Line Items]        
Accumulated other comprehensive loss, net of tax $ 46us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax $ 34us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax $ 48us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax $ 63us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Pension Benefits [Member]        
Pension Plans, Postretirement and Other Employee Benefits [Line Items]        
Accumulated other comprehensive loss, net of tax $ 37us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
$ 25us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember