Pension and Other Postretirement Benefits - Amounts Recognized in Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset	$ 2,246	$ 2,409
Current liabilities	(2,308)	(2,056)
Pension benefit obligation	(466)	(396)
Other postretirement benefit obligations	(215)	(265)
Deferred income tax liabilities	(3,393)	(3,242)
Deferred income tax liabilities	15	17
Accumulated other comprehensive loss, net of tax	36	46	$ 34	$ 48
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset	870	871
Current liabilities	(6)	(6)
Pension benefit obligation	(466)	(396)
Deferred income tax liabilities	(162)	(193)
Accumulated other comprehensive loss, net of tax	28	37
Net amount recorded	264	313
Other Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset	40	75
Other postretirement benefit obligations	(215)	(265)
Deferred income tax liabilities	70	77
Net amount recorded	$ (105)	$ (113)