Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income (Numerator):
Net income	$ 163	$ 236
Weighted average number of shares:
Weighted average shares outstanding for computing basic earnings per common share (in shares)	540,458	538,799
Incremental common shares attributable to dilutive securities:
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	541,522	540,187
Basic earnings per common share (in dollars per share)	$ 0.30	$ 0.44
Diluted earnings per common share (in dollars per share)	$ 0.30	$ 0.44
Common Class A
Income (Numerator):
Net income	$ 163	$ 236
Earnings applicable to non-vested restricted stock	0	0
Net income applicable to common stock for computing basic earnings per common share	163	236
Net income as adjusted for purposes of computing diluted earnings per common share	$ 163	$ 236
Weighted average number of shares:
Outstanding during period (in shares)	547,618	544,845
Non-vested restricted stock (in shares)	(7,160)	(6,046)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	540,458	538,799
Incremental common shares attributable to dilutive securities:
Shares issuable under convertible securities (in shares)	10	10
Shares issuable under incentive compensation plans (in shares)	1,054	1,378
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	541,522	540,187
Basic earnings per common share (in dollars per share)	$ 0.30	$ 0.44
Diluted earnings per common share (in dollars per share)	$ 0.30	$ 0.44
Number of shares of common stock excluded from the computation of diluted earnings per share (in shares)	4,300	3,100