Pension and Other Postretirement Benefits - Changes in Plan Assets and Benefit Obligations Recognized in AOCL and Regulatory Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits [Member]
Amounts amortized during the year:
Amortization of prior service (cost) credit	$ (2)	$ (2)	$ (2)
Amortization of net actuarial loss	(65)	(45)	(67)
Amounts arising during the year:
Current year prior service cost (credit)			3
Current year actuarial loss (gain)	50	276	(218)
Total recognized in AOCL and Regulatory assets for the year ended December 31	(17)	229	(284)
Other Postretirement Benefits [Member]
Amounts amortized during the year:
Amortization of prior service (cost) credit	13	13	11
Amortization of net actuarial loss	(8)	(3)	(12)
Amounts arising during the year:
Current year prior service cost (credit)			(124)
Current year actuarial loss (gain)	(39)	62	(109)
Total recognized in AOCL and Regulatory assets for the year ended December 31	$ (34)	$ 72	$ (234)