Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income (Numerator):
Net income	$ 92	$ 152	$ 272	$ 584
Weighted average number of shares:
Weighted average shares outstanding for computing basic earnings per common share (in shares)	541,521	539,806	541,113	539,411
Incremental common shares attributable to dilutive securities:
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	541,963	540,917	541,879	540,493
Basic earnings per common share (in dollars per share)	$ 0.17	$ 0.28	$ 0.50	$ 1.08
Diluted earnings per common share (in dollars per share)	$ 0.17	$ 0.28	$ 0.50	$ 1.08
Common Class A
Income (Numerator):
Net income	$ 92	$ 152	$ 272	$ 584
Earnings applicable to non-vested restricted stock	0	0	0	0
Net income applicable to common stock for computing basic earnings per common share	92	152	272	584
Net income as adjusted for purposes of computing diluted earnings per common share	$ 92	$ 152	$ 272	$ 584
Weighted average number of shares:
Outstanding during period (in shares)	549,618	546,310	548,779	545,715
Non-vested restricted stock (in shares)	(8,097)	(6,504)	(7,666)	(6,304)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	541,521	539,806	541,113	539,411
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)	432	1,101	756	1,072
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	541,963	540,917	541,879	540,493
Basic earnings per common share (in dollars per share)	$ 0.17	$ 0.28	$ 0.50	$ 1.08
Diluted earnings per common share (in dollars per share)	$ 0.17	$ 0.28	$ 0.50	$ 1.08
Number of shares of common stock excluded from the computation of diluted earnings per share (in shares)	5,100	2,700	4,200	3,300