Accumulated Other Comprehensive Income - Reclassification of Accumulated Other Comprehensive Income with Cash Flow Derivative Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense	$ (95)	$ (116)	$ (294)	$ (352)
Other, net	(1)	(2)	(2)	(7)
Operating revenues	3,360	3,602	9,715	10,559
Income before income taxes	496	414	663	1,152
Tax (expense) benefit	(159)	(133)	(184)	(412)
Consolidated net income (loss)	337	281	479	740
Amount Reclassified from Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	15	13	28	(2)
Tax (expense) benefit	(6)	(5)	(11)	1
Consolidated net income (loss)	9	8	17	(1)
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	(4)	(3)	(9)	(8)
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	$ 19	$ 16	$ 40	16
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)