Schedule of Selected Quarterly Financial Data (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Selected Quarterly Financial Data [Line Items]
Operating revenue	$ 9,329	$ 10,228	$ 10,001	$ 8,721	$ 8,702	$ 9,909	$ 9,939	$ 8,602	$ 38,279	$ 37,152	$ 37,110
Income (loss) from operations	235	508	770	(264)	(465)	200	575	(271)	1,249	39	1,822
Net income (loss)	$ 140	$ 379	$ 469	$ (417)	$ (620)	$ 6	$ 339	$ (448)	$ 571	$ (723)	$ 840
Basic earnings (loss) per share	$ 0.39	$ 1.06	$ 1.37	$ (1.26)	$ (1.87)	$ 0.02	$ 1.02	$ (1.36)	$ 1.64	$ (2.18)	$ 2.54
Diluted earnings (loss) per share	$ 0.37	$ 0.98	$ 1.21	$ (1.26)	$ (1.87)	$ 0.02	$ 0.89	$ (1.36)	$ 1.53	$ (2.18)	$ 2.26