Accumulated Other Comprehensive Income - Reclassification of Accumulated Other Comprehensive Income with Cash Flow Derivative Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other, net	$ (1)	$ (2)	$ (1)	$ (5)
Operating revenues	3,315	3,561	6,355	6,957
Provision for (benefit from) income taxes	(116)	(180)	(25)	(279)
Consolidated net income	273	222	142	459
Amount Reclassified from Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gains and losses on cash flow hedges, Total before tax	(7)	(17)	13	(15)
Provision for (benefit from) income taxes	3	7	(5)	6
Consolidated net income	(4)	(10)	8	(9)
Interest Rate Swaps [Member] | Amount Reclassified from Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense	(2)	(4)	(5)	(5)
Treasury Rate Locks [Member] | Amount Reclassified from Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense			(3)	(1)
Foreign Currency Derivatives [Member] | Amount Reclassified from Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other, net	$ (5)	$ (13)	$ 21
Electricity Commodity Derivatives [Member] | Amount Reclassified from Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating revenues				$ (9)