Pension and Other Postretirement Benefits - Amounts Recognized in Consolidated Balance Sheets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset	$ 2,409	$ 2,087
Current liabilities	(2,065)	(2,313)
Pension benefit obligation	(396)	(116)
Other postretirement benefit obligations	(265)	(206)
Deferred income taxes liabilities	3,266	2,928
Accumulated other comprehensive loss net of tax	46	34	48	63
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset	871	664
Current liabilities	(6)	(6)
Pension benefit obligation	(396)	(116)
Deferred income taxes liabilities	(193)	(217)
Accumulated other comprehensive loss net of tax	37	25
Net amount recorded	313	350
Other Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset	75	3
Other postretirement benefit obligations	(265)	(206)
Deferred income taxes liabilities	77	82
Net amount recorded	$ (113)	$ (121)