Quarterly Financial Data (Unaudited) - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 9,458	$ 10,483	$ 10,447	$ 9,251	$ 9,502	$ 11,107	$ 10,707	$ 9,388	$ 39,639	$ 40,704	$ 40,362
Operating income	1,020	1,969	2,423	1,540	1,717	2,213	2,474	1,398	6,952	7,802	2,206
Net Income	$ 622	$ 1,259	$ 1,546	$ 946	$ 980	$ 1,315	$ 1,485	$ 746	$ 4,373	$ 4,526	$ 659
Basic earnings per share (usd per share)	$ 0.85	$ 1.70	$ 2.04	$ 1.22	$ 1.26	$ 1.67	$ 1.85	$ 0.91	$ 5.82	$ 5.68	$ 0.79
Diluted earnings per share (usd per share)	$ 0.84	$ 1.69	$ 2.03	$ 1.21	$ 1.25	$ 1.65	$ 1.83	$ 0.90	$ 5.79	$ 5.63	$ 0.78