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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Common Share [Abstract]
Net income		$ 522	$ 503	$ 452	$ 412	$ 416	$ 376	$ 353	$ 302	$ 1,889	$ 1,447	$ 1,321
Preferred stock dividends and other	[1]									(143)	(83)	(60)
Net Income Available to Common Stockholders		$ 478	$ 470	$ 406	$ 392	$ 378	$ 365	$ 330	$ 291	$ 1,746	$ 1,364	$ 1,261
Weighted Average Common Shares Outstanding - Basic		1,329	1,324	1,322	1,321	1,319	1,316	1,314	1,312	1,324	1,315	1,303
Common stock equivalent shares related to stock incentive plans										10	12	12
Weighted-average common shares outstanding — diluted		1,341	1,334	1,333	1,330	1,330	1,328	1,326	1,323	1,334	1,327	1,315
Basic EPS		$ 0.36	$ 0.36	$ 0.31	$ 0.30	$ 0.29	$ 0.28	$ 0.25	$ 0.22	$ 1.32	$ 1.04	$ 0.96
Diluted EPS		$ 0.36	$ 0.35	$ 0.30	$ 0.29	$ 0.28	$ 0.28	$ 0.25	$ 0.22	$ 1.31	$ 1.03	$ 0.95
Antidilutive stock options and restricted stock units excluded from the calculation of diluted EPS										26	23	24

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