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, 2013	Dec. 31, 2012	Dec. 31, 2011
Pension Benefits [Member]
Amounts amortized during the year:
Amortization of prior service (cost) credit	$ (2)	$ (1)
Amortization of net actuarial (loss)	(67)	(64)	(47)
Amounts arising during the year:
Current year prior service cost (credit)	3		19
Current year actuarial (gain) loss	(218)	220	177
Total recognized in AOCL and Regulatory assets for the year ended December 31	(284)	155	149
Other Postretirement Benefits [Member]
Amounts amortized during the year:
Amortization of prior service (cost) credit	11	4	5
Amortization of net actuarial (loss)	(12)	(14)	(14)
Amounts arising during the year:
Current year prior service cost (credit)	(124)		6
Current year actuarial (gain) loss	(109)	4	53
Total recognized in AOCL and Regulatory assets for the year ended December 31	$ (234)	$ (6)	$ 50