Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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
Earnings Per Share [Abstract]
Net income	$ 572	$ 1,178	$ 1,224	$ 603	$ 622	$ 1,259	$ 1,546	$ 946	$ 3,577	$ 4,373	$ 4,526
Basic weighted average shares outstanding (shares)									720	751	797
Dilutive effect of share-based awards (shares)									3	4	7
Diluted weighted average shares outstanding (shares)									723	755	804
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