Earnings per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income (Numerator):
Net income	$ 196	$ 143	$ 432	$ 335
Weighted average number of shares:
Weighted average shares outstanding for computing basic earnings per common share (in shares)	539,627	558,640	539,213	560,304
Incremental common shares attributable to dilutive securities:
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	540,375	559,220	540,281	561,362
Basic earnings per common share (in dollars per share)	$ 0.36	$ 0.26	$ 0.80	$ 0.60
Diluted earnings per common share (in dollars per share)	$ 0.36	$ 0.26	$ 0.80	$ 0.60
Number of shares of common stock excluded from the computation of diluted earnings per share (in shares)	4,100	2,400	3,600	2,300
Common Class A
Income (Numerator):
Net income	$ 196	$ 143	$ 432	$ 335
Earnings applicable to non-vested restricted stock	0	0	0	0
Net income applicable to common stock for computing basic earnings per common share	196	143	432	335
Net income as adjusted for purposes of computing diluted earnings per common share	$ 196	$ 143	$ 432	$ 335
Weighted average number of shares:
Outstanding during period (in shares)	545,988	563,495	545,417	565,091
Non-vested restricted stock (in shares)	(6,361)	(4,855)	(6,204)	(4,787)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	539,627	558,640	539,213	560,304
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)	738	570	1,058	1,048
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	540,375	559,220	540,281	561,362
Basic earnings per common share (in dollars per share)	$ 0.36	$ 0.26	$ 0.80	$ 0.60
Diluted earnings per common share (in dollars per share)	$ 0.36	$ 0.26	$ 0.80	$ 0.60