Earnings (Loss) Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
(Loss) Income (Numerator):
Net income (loss)	$ 302	$ 272	$ (5,492)	$ 679
Weighted-average number of shares:
Weighted average shares outstanding for computing basic earnings per common share (in shares)	1,072,543	1,066,904	1,070,921	1,065,410
Incremental common shares attributable to dilutive securities:
Number of shares as adjusted for purposes of computing diluted (loss) earnings per common share (in shares)	1,074,790	1,072,351	1,070,921	1,069,726
Basic (loss) earnings per common share (in dollars per share)	$ 0.28	$ 0.25	$ (5.13)	$ 0.64
Diluted (loss) earnings per common share (in dollars per share)	$ 0.28	$ 0.25	$ (5.13)	$ 0.63
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
Common Class A
(Loss) Income (Numerator):
Net income (loss)		$ 272		$ 679
Net income (loss) applicable to common stock for computing basic earnings per common share	$ 302	272	$ (5,492)	679
Net income (loss) as adjusted for purposes of computing diluted earnings per common share	$ 302	$ 272	$ (5,492)	$ 679
Weighted-average number of shares:
Outstanding during period (in shares)	1,090,755	1,080,589	1,088,229	1,077,712
Non-vested restricted stock (in shares)	(18,212)	(13,685)	(17,308)	(12,302)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	1,072,543	1,066,904	1,070,921	1,065,410
Incremental common shares attributable to dilutive securities:
Shares issuable under convertible securities (in shares)	10	10	0	10
Shares issuable under incentive compensation plans (in shares)	2,237	5,437	0	4,306
Number of shares as adjusted for purposes of computing diluted (loss) earnings per common share (in shares)	1,074,790	1,072,351	1,070,921	1,069,726
Basic (loss) earnings per common share (in dollars per share)	$ 0.28	$ 0.25	$ (5.13)	$ 0.64
Diluted (loss) earnings per common share (in dollars per share)	$ 0.28	$ 0.25	$ (5.13)	$ 0.63
Number of shares of common stock excluded from the computation of diluted earnings per share (in shares)	5,200	1,500	8,300	3,000