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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Data [Line Items]
Operating revenue	$ 9,313	$ 10,563	$ 10,329	$ 8,696	$ 9,329	$ 10,228	$ 10,001	$ 8,721	$ 38,901	$ 38,279	$ 37,152
Income (loss) from operations	625	1,191	906	(349)	235	508	770	(264)	2,373	1,249	39
Net income (loss)	$ 28	$ 924	$ 789	$ (609)	$ 140	$ 379	$ 469	$ (417)	$ 1,132	$ 571	$ (723)
Basic earnings (loss) per share	$ 0.08	$ 2.49	$ 2.11	$ (1.66)	$ 0.39	$ 1.06	$ 1.37	$ (1.26)	$ 3.05	$ 1.64	$ (2.18)
Diluted earnings (loss) per share	$ 0.07	$ 2.37	$ 2.01	$ (1.66)	$ 0.37	$ 0.98	$ 1.21	$ (1.26)	$ 2.93	$ 1.53	$ (2.18)