Earnings Per Common Share (EPS under Basic and Diluted Computations) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Common Share [Abstract]
Net income		$ 416	$ 376	$ 353	$ 302	$ 350	$ 321	$ 324	$ 326	$ 1,447	$ 1,321	$ 1,071
Preferred stock dividends and other	[1]									(83)	(60)	(61)
Net income available to common stockholders		$ 378	$ 365	$ 330	$ 291	$ 329	$ 312	$ 302	$ 318	$ 1,364	$ 1,261	$ 1,010
Weighted Average Common Shares Outstanding - Basic		1,319	1,316	1,314	1,312	1,309	1,304	1,302	1,299	1,315	1,303	1,285
Common stock equivalent shares related to stock incentive plans										12	12	8
Weighted-average common shares outstanding — diluted		1,330	1,328	1,326	1,323	1,320	1,316	1,313	1,311	1,327	1,315	1,293
Basic EPS		$ 0.29	$ 0.28	$ 0.25	$ 0.22	$ 0.25	$ 0.24	$ 0.23	$ 0.24	$ 1.04	$ 0.96	$ 0.78
Diluted EPS		$ 0.28	$ 0.28	$ 0.25	$ 0.22	$ 0.25	$ 0.24	$ 0.23	$ 0.24	$ 1.03	$ 0.95	$ 0.78
Antidilutive stock options and restricted stock awards excluded from the calculation of diluted EPS										23	24	34

[1] Includes preferred stock dividends and undistributed earnings and dividends allocated to non-vested restricted stock units.