Earnings per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income (Numerator):
Net income	$ 143	$ 193	$ 335	$ 396
Earnings applicable to non-vested restricted stock	0	0	0	0
Net income applicable to common stock for computing basic earnings per common share	143	193	335	396
Net income as adjusted for purposes of computing diluted earnings per common share	$ 143	$ 193	$ 335	$ 396
Weighted average number of shares:
Outstanding during period (in shares)	563,495	572,240	565,091	575,218
Non-vested restricted stock (in shares)	(4,855)	(4,325)	(4,787)	(3,993)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	558,640	567,915	560,304	571,225
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)	570	1,107	1,048	1,009
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	559,220	569,032	561,362	572,244
Basic earnings per common share:
Basic earnings per common share (in dollars per share)	$ 0.26	$ 0.34	$ 0.60	$ 0.69
Diluted earnings per common share:
Diluted earnings per common share (in dollars per share)	$ 0.26	$ 0.34	$ 0.60	$ 0.69
Stock option awards
Diluted earnings per common share:
Number of shares of common stock excluded from the computation of diluted earnings per share (in shares)	2,400	2,400	2,300	2,800