Pension and Other Postretirement Benefits - Amounts Recognized in Consolidated Balance Sheets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset	$ 2,087	$ 2,614
Current liabilities	(2,313)	(2,543)
Pension benefit obligation	(116)	(449)
Other postretirement benefit obligations	(206)	(454)
Accumulated other comprehensive loss net of tax	34	48
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset	664	934
Current liabilities	(6)	(6)
Pension benefit obligation	(116)	(449)
Other postretirement benefit obligations
Deferred income tax liabilities, net	(217)	(216)
Accumulated other comprehensive loss net of tax	25	32
Net amount recorded	350	295
Other Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset	3	237
Current liabilities
Pension benefit obligation
Other postretirement benefit obligations	(206)	(454)
Deferred income tax liabilities, net	82	88
Accumulated other comprehensive loss net of tax
Net amount recorded	$ (121)	$ (129)