Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
(Loss) Income (Numerator):
Net income	$ 475	$ 314
Net income applicable to common stock for computing basic earnings per common share	475	314
Net income as adjusted for purposes of computing diluted earnings per common share	$ 475	$ 314
Weighted-average number of shares:
Outstanding during period (in shares)	1,100,350	1,092,970
Non-vested restricted stock (in shares)	(17,876)	(17,511)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	1,082,474	1,075,459
Incremental common shares attributable to dilutive securities:
Shares issuable under convertible securities (in shares)	10	10
Shares issuable under incentive compensation plans (in shares)	9,102	6,285
Number of shares as adjusted for purposes of computing diluted earnings (loss) per common share (in shares)	1,091,586	1,081,754
Basic earnings per common share (in dollars per share)	$ 0.44	$ 0.29
Diluted earnings per common share (in dollars per share)	$ 0.44	$ 0.29
Number of shares of common stock excluded from the computation of diluted earnings per share (less than) (in shares)	1,000	2,700