Quarterly Financial Data (Unaudited) - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 9,502	$ 11,107	$ 10,707	$ 9,388	$ 9,647	$ 11,178	$ 10,621	$ 8,916	$ 40,704	$ 40,362	$ 37,773
Operating income	1,717	2,213	2,474	1,398	(828)	835	1,579	620	7,802	2,206	3,400
Net Income	$ 980	$ 1,315	$ 1,485	$ 746	$ (712)	$ 357	$ 801	$ 213	$ 4,526	$ 659	$ 10,540
Basic earnings per share (usd per share)	$ 1.26	$ 1.67	$ 1.85	$ 0.91	$ (0.86)	$ 0.43	$ 0.95	$ 0.25	$ 5.68	$ 0.79	$ 12.41
Diluted earnings per share (usd per share)	$ 1.25	$ 1.65	$ 1.83	$ 0.90	$ (0.86)	$ 0.42	$ 0.94	$ 0.25	$ 5.63	$ 0.78	$ 12.29