Pension and Other Postretirement Benefits - Changes in Plan Assets and Benefit Obligations Recognized 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 Benefits [Member]
Amounts amortized during the year:
Amortization of prior service (cost) credit	$ (2)	$ (2)	$ (1)
Amortization of net actuarial (loss)	(45)	(67)	(64)
Amounts arising during the year:
Current year prior service cost (credit)		3
Current year actuarial loss (gain)	276	(218)	220
Total recognized in AOCL and Regulatory assets for the year ended December 31	229	(284)	155
Other Postretirement Benefits [Member]
Amounts amortized during the year:
Amortization of prior service (cost) credit	13	11	4
Amortization of net actuarial (loss)	(3)	(12)	(14)
Amounts arising during the year:
Current year prior service cost (credit)		(124)
Current year actuarial loss (gain)	62	(109)	4
Total recognized in AOCL and Regulatory assets for the year ended December 31	$ 72	$ (234)	$ (6)