Accumulated Other Comprehensive Income - Reclassification of Component of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense, net									$ (385)	$ (466)	$ (477)
Other, net									(7)	(29)	(74)
Operating revenues	$ 3,246	$ 3,360	$ 3,315	$ 3,040	$ 3,437	$ 3,602	$ 3,561	$ 3,396	12,961	13,996	13,983
Income before income taxes									1,060	1,751	494
Tax (expense) benefit									(308)	(413)	(364)
Total reclassifications for the period, Net of tax	$ 273	$ 337	$ 273	$ (131)	$ 598	$ 281	$ 222	$ 237	752	1,338	130
Amount Reclassified from Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes									35	8	3
Tax (expense) benefit									(14)	(3)	(1)
Total reclassifications for the period, Net of tax									21	5	2
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, net									(12)	(10)	(7)
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, net									(4)	(1)	(2)
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									$ 51	27	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										$ (8)	$ (9)