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Pension and Other Postretirement Benefits - Schedule of Amounts Included in AOCL and Regulatory Assets (Parenthetical) (Detail) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Accumulated other comprehensive loss, net of tax $ 36 $ 46 $ 34 $ 48
Pension Benefits [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Accumulated other comprehensive loss, net of tax $ 28 $ 37