Quarterly Financial Data (Unaudited) - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 10,245	$ 11,060	$ 10,791	$ 9,148	$ 9,458	$ 10,483	$ 10,447	$ 9,251	$ 41,244	$ 39,639	$ 40,704
Operating income	1,193	1,839	2,028	1,053	1,020	1,969	2,423	1,540	6,114	6,952	7,802
Net Income	$ 572	$ 1,178	$ 1,224	$ 603	$ 622	$ 1,259	$ 1,546	$ 946	$ 3,577	$ 4,373	$ 4,526
Basic earnings per share (usd per share)	$ 0.81	$ 1.64	$ 1.68	$ 0.83	$ 0.85	$ 1.70	$ 2.04	$ 1.22	$ 4.97	$ 5.82	$ 5.68
Diluted earnings per share (usd per share)	$ 0.80	$ 1.64	$ 1.68	$ 0.82	$ 0.84	$ 1.69	$ 2.03	$ 1.21	$ 4.95	$ 5.79	$ 5.63