Pension and Other Postretirement Benefits - Schedule of Amounts Included in AOCL and Regulatory Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory assets	$ 2,246	$ 2,409
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Unrecognized net actuarial loss	910	925
Unamortized prior service cost (credit)	6	8
Total	916	933
Accumulated other comprehensive loss ($28 million and $37 million, net of tax, at December 31, 2015 and 2014, respectively)	46	62
Regulatory assets	870	871
Total	916	933
Other Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Unrecognized net actuarial loss	128	176
Unamortized prior service cost (credit)	(88)	(101)
Total	40	75
Regulatory assets	40	75
Total	$ 40	$ 75