Earnings per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income (Numerator):
Net income	$ 205	$ 188	$ 540	$ 584
Earnings applicable to non-vested restricted stock	0	0	0	0
Net income applicable to common stock for computing basic earnings per common share	205	188	540	584
Net income as adjusted for purposes of computing diluted earnings per common share	$ 205	$ 188	$ 540	$ 584
Weighted average number of shares:
Outstanding during period (in shares)	559,991	570,545	563,391	573,661
Non-vested restricted stock (in shares)	(5,094)	(4,580)	(4,889)	(4,189)
Weighted average shares outstanding for computing basic earnings per common share (in shares)	554,897	565,965	558,502	569,472
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)	249	1,457	781	1,158
Number of shares as adjusted for purposes of computing diluted earnings per common share (in shares)	555,156	567,432	559,293	570,640
Basic earnings per common share:
Basic earnings per common share (in dollars per share)	$ 0.37	$ 0.33	$ 0.97	$ 1.03
Diluted earnings per common share:
Diluted earnings per common share (in dollars per share)	$ 0.37	$ 0.33	$ 0.97	$ 1.02
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)	3,100	2,100	2,500	2,500