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, 2013	Dec. 31, 2012	Dec. 31, 2011
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Accumulated other comprehensive loss ($25 million and $32 million, net of tax, at December 31, 2013 and 2012, respectively)	$ (13)	$ 14	$ 11
Regulatory assets	2,087	2,614
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Unrecognized net actuarial loss	694	979
Unamortized prior service cost (credit)	10	9
Total	704	988
Accumulated other comprehensive loss ($25 million and $32 million, net of tax, at December 31, 2013 and 2012, respectively)	40	54
Regulatory assets	664	934
Total	704	988
Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Unrecognized net actuarial loss	117	238
Unamortized prior service cost (credit)	(114)	(1)
Total	3	237
Accumulated other comprehensive loss ($25 million and $32 million, net of tax, at December 31, 2013 and 2012, respectively)
Regulatory assets	3	237
Total	$ 3	$ 237