Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share Reconciliation [Abstract]
Net income	$ 1,310	$ 827	$ (40)	$ 600	$ 873	$ 396	$ 557	$ 796	$ 2,697	$ 2,622	$ 2,523
Participating securities — allocated income									(19)	(19)	(19)
Net income available to common shareholders -- basic									2,678	2,603	2,504
Net income available to common shareholders -- diluted									$ 2,678	$ 2,603	$ 2,504
Weighted average shares outstanding, basic (in shares)									253.5	260.0	267.4
Weighted average effects of dilutive securities - stock options and performance shares (in shares)									1.1	2.3	2.4
Weighted average shares outstanding, diluted (in shares)									254.6	262.3	269.8
Net income per common share, basic (in dollars per share)	$ 5.13	$ 3.24	$ (0.16)	$ 2.34	$ 3.37	$ 1.52	$ 2.11	$ 3.01	$ 10.56	$ 10.01	$ 9.37
Net income per common share, diluted (in dollars per share)	$ 5.10	$ 3.23	$ (0.16)	$ 2.33	$ 3.35	$ 1.50	$ 2.10	$ 2.99	$ 10.52	$ 9.92	$ 9.28