Earnings (Loss) Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
(Loss) Income (Numerator):
Net income (loss)	$ 371	$ 292	$ (5,794)	$ 407
Weighted-average number of shares:
Weighted average shares outstanding for computing basic earnings per common share (in shares)	1,071,341	1,064,711	1,070,110	1,064,663
Incremental common shares attributable to dilutive securities:
Number of shares as adjusted for purposes of computing diluted (loss) earnings per common share (in shares)	1,072,813	1,068,819	1,070,110	1,068,414
Basic (loss) earnings per common share (in dollars per share)	$ 0.35	$ 0.27	$ (5.41)	$ 0.38
Diluted (loss) earnings per common share (in dollars per share)	$ 0.35	$ 0.27	$ (5.41)	$ 0.38
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)			2,400
Common Class A
(Loss) Income (Numerator):
Net income (loss)	$ 371	$ 292	$ (5,794)	$ 407
Net income (loss) applicable to common stock for computing basic earnings per common share	371	292	(5,794)	407
Net income (loss) as adjusted for purposes of computing diluted earnings per common share	$ 371	$ 292	$ (5,794)	$ 407
Weighted-average number of shares:
Outstanding during period (in shares)	1,090,342	1,078,986	1,086,966	1,076,273
Non-vested restricted stock (in shares)	(19,001)	(14,275)	(16,856)	(11,610)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	1,071,341	1,064,711	1,070,110	1,064,663
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)	1,462	4,098	0	3,741
Number of shares as adjusted for purposes of computing diluted (loss) earnings per common share (in shares)	1,072,813	1,068,819	1,070,110	1,068,414
Basic (loss) earnings per common share (in dollars per share)	$ 0.35	$ 0.27	$ (5.41)	$ 0.38
Diluted (loss) earnings per common share (in dollars per share)	$ 0.35	$ 0.27	$ (5.41)	$ 0.38
Number of shares of common stock excluded from the computation of diluted earnings per share (in shares)	14,300	3,100	9,900	3,700