Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income (Numerator):
Net income	$ 152	$ 205	$ 584	$ 540
Weighted average number of shares:
Weighted average shares outstanding for computing basic earnings per common share (in shares)	539,806	554,897	539,411	558,502
Incremental common shares attributable to dilutive securities:
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	540,917	555,156	540,493	559,293
Basic earnings per common share (in dollars per share)	$ 0.28	$ 0.37	$ 1.08	$ 0.97
Diluted earnings per common share (in dollars per share)	$ 0.28	$ 0.37	$ 1.08	$ 0.97
Common Class A
Income (Numerator):
Net income	$ 152	$ 205	$ 584	$ 540
Earnings applicable to non-vested restricted stock	0	0	0	0
Net income applicable to common stock for computing basic earnings per common share	152	205	584	540
Net income as adjusted for purposes of computing diluted earnings per common share	$ 152	$ 205	$ 584	$ 540
Weighted average number of shares:
Outstanding during period (in shares)	546,310	559,991	545,715	563,391
Non-vested restricted stock (in shares)	(6,504)	(5,094)	(6,304)	(4,889)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	539,806	554,897	539,411	558,502
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)	1,101	249	1,072	781
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	540,917	555,156	540,493	559,293
Basic earnings per common share (in dollars per share)	$ 0.28	$ 0.37	$ 1.08	$ 0.97
Diluted earnings per common share (in dollars per share)	$ 0.28	$ 0.37	$ 1.08	$ 0.97
Number of shares of common stock excluded from the computation of diluted earnings per share (in shares)	2,700	3,100	3,300	2,500