Earnings per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Income (Numerator):
Net income	$ 236	$ 192
Earnings applicable to non-vested restricted stock	0	0
Net income applicable to common stock for computing basic earnings per common share	236	192
Net income as adjusted for purposes of computing diluted earnings per common share	$ 236	$ 192
Weighted average number of shares:
Weighted average shares outstanding for computing basic earnings per common share (in shares)	538,799	561,969
Incremental common shares attributable to dilutive securities:
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	540,187	563,505
Basic earnings per common share (in dollars per share)	$ 0.44	$ 0.34
Diluted earnings per common share (in dollars per share)	$ 0.44	$ 0.34
Common Class A
Weighted average number of shares:
Outstanding during period (in shares)	544,845	566,687
Non-vested restricted stock (in shares)	(6,046)	(4,718)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	538,799	561,969
Incremental common shares attributable to dilutive securities:
Shares issuable under convertible securities (in shares)	10	10
Shares issuable under incentive compensation plans (in shares)	1,378	1,526
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	540,187	563,505
Basic earnings per common share (in dollars per share)	$ 0.44	$ 0.34
Diluted earnings per common share (in dollars per share)	$ 0.44	$ 0.34
Number of shares of common stock excluded from the computation of diluted earnings per share (in shares)	3,100	2,200