Earnings (Loss) Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
(Loss) Income (Numerator):
Net income (loss)	$ 366	$ 302	$ 1,057	$ (5,492)
Net income (loss) applicable to common stock for computing basic earnings per common share	366	302	1,057	(5,492)
Net income (loss) as adjusted for purposes of computing diluted earnings per common share	$ 366	$ 302	$ 1,057	$ (5,492)
Weighted-average number of shares:
Outstanding during period (in shares)	1,097,496	1,090,755	1,096,017	1,088,229
Non-vested restricted stock (in shares)	(16,991)	(18,212)	(17,345)	(17,308)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	1,080,505	1,072,543	1,078,672	1,070,921
Incremental common shares attributable to dilutive securities:
Shares issuable under convertible securities (in shares)	10	10	10	0
Shares issuable under incentive compensation plans (in shares)	5,151	2,237	4,686	0
Number of shares as adjusted for purposes of computing diluted earnings (loss) per common share (in shares)	1,085,666	1,074,790	1,083,368	1,070,921
Basic earnings (loss) per common share (in dollars per share)	$ 0.34	$ 0.28	$ 0.98	$ (5.13)
Diluted earnings (loss) per common share (in dollars per share)	$ 0.34	$ 0.28	$ 0.98	$ (5.13)
Number of shares of common stock excluded from the computation of diluted earnings per share (in shares)	400	5,200	4,000	8,300
Convertible Debt Securities
Incremental common shares attributable to dilutive securities:
Number of shares of common stock excluded from the computation of diluted earnings per share (in shares)				2,300