Pension and Other Postretirement Benefits - Schedule of Amounts Included in AOCL and Regulatory Assets (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Accumulated other comprehensive loss ($37 million and $25 million, net of tax, at December 31, 2014 and 2013, respectively)	$ (20)	$ 13	$ (14)
Regulatory assets	2,409	2,087
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Unrecognized net actuarial loss	925	694
Unamortized prior service cost (credit)	8	10
Total	933	704
Accumulated other comprehensive loss ($37 million and $25 million, net of tax, at December 31, 2014 and 2013, respectively)	62	40
Regulatory assets	871	664
Total	933	704
Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Unrecognized net actuarial loss	176	117
Unamortized prior service cost (credit)	(101)	(114)
Total	75	3
Regulatory assets	75	3
Total	$ 75	$ 3