Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income (Numerator):
Net income	$ 272	$ 92	$ 679	$ 272
Weighted-average number of shares:
Weighted average shares outstanding for computing basic earnings per common share (in shares)	1,066,904	541,521	1,065,410	541,113
Incremental common shares attributable to dilutive securities:
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	1,072,351	541,963	1,069,726	541,879
Basic earnings per common share (in dollars per share)	$ 0.25	$ 0.17	$ 0.64	$ 0.50
Diluted earnings per common share (in dollars per share)	$ 0.25	$ 0.17	$ 0.63	$ 0.50
Common Class A
Income (Numerator):
Net income	$ 272	$ 92	$ 679	$ 272
Earnings applicable to non-vested restricted stock	0	0	0	0
Net income applicable to common stock for computing basic earnings per common share	272	92	679	272
Net income as adjusted for purposes of computing diluted earnings per common share	$ 272	$ 92	$ 679	$ 272
Weighted-average number of shares:
Outstanding during period (in shares)	1,080,589	549,618	1,077,712	548,779
Non-vested restricted stock (in shares)	(13,685)	(8,097)	(12,302)	(7,666)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	1,066,904	541,521	1,065,410	541,113
Incremental common shares attributable to dilutive securities:
Shares issuable under convertible securities (in shares)	10	10	10	10
Shares issuable under incentive compensation plans (in shares)	5,437	432	4,306	756
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	1,072,351	541,963	1,069,726	541,879
Basic earnings per common share (in dollars per share)	$ 0.25	$ 0.17	$ 0.64	$ 0.50
Diluted earnings per common share (in dollars per share)	$ 0.25	$ 0.17	$ 0.63	$ 0.50
Number of shares of common stock excluded from the computation of diluted earnings per share (in shares)	1,500	5,100	3,000	4,200