Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income (Numerator):
Net income	$ 17	$ 196	$ 180	$ 432
Weighted average number of shares:
Weighted average shares outstanding for computing basic earnings per common share (in shares)	541,361	539,627	540,909	539,213
Incremental common shares attributable to dilutive securities:
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	542,151	540,375	541,836	540,281
Basic earnings per common share (in dollars per share)	$ 0.03	$ 0.36	$ 0.33	$ 0.80
Diluted earnings per common share (in dollars per share)	$ 0.03	$ 0.36	$ 0.33	$ 0.80
Common Class A
Income (Numerator):
Net income	$ 17	$ 196	$ 180	$ 432
Earnings applicable to non-vested restricted stock	0	0	0	0
Net income applicable to common stock for computing basic earnings per common share	17	196	180	432
Net income as adjusted for purposes of computing diluted earnings per common share	$ 17	$ 196	$ 180	$ 432
Weighted average number of shares:
Outstanding during period (in shares)	549,100	545,988	548,359	545,417
Non-vested restricted stock (in shares)	(7,739)	(6,361)	(7,450)	(6,204)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	541,361	539,627	540,909	539,213
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)	780	738	917	1,058
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	542,151	540,375	541,836	540,281
Basic earnings per common share (in dollars per share)	$ 0.03	$ 0.36	$ 0.33	$ 0.80
Diluted earnings per common share (in dollars per share)	$ 0.03	$ 0.36	$ 0.33	$ 0.80
Number of shares of common stock excluded from the computation of diluted earnings per share (in shares)	3,100	4,100	3,700	3,600