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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Earnings Per Share [Abstract]
Net income		$ 597	$ 618	$ 575	$ 564	$ 522	$ 503	$ 452	$ 412	$ 2,354		$ 1,889		$ 1,447
Preferred stock dividends and other	[1]									(174)		(143)		(83)
Net Income Available to Common Stockholders		$ 550	$ 575	$ 530	$ 525	$ 478	$ 470	$ 406	$ 392	$ 2,180		$ 1,746		$ 1,364
Weighted Average Common Shares Outstanding - Basic (in shares)		1,343	1,339	1,338	1,336	1,329	1,324	1,322	1,321	1,339		1,324		1,315
Common stock equivalent shares related to stock incentive plans (in shares)										14		10		12
Weighted-average common shares outstanding — diluted (in shares)		1,358	1,353	1,351	1,351	1,341	1,334	1,333	1,330	1,353	[2]	1,334	[2]	1,327	[2]
Basic EPS (USD per share)		$ 0.41	$ 0.43	$ 0.40	$ 0.39	$ 0.36	$ 0.36	$ 0.31	$ 0.30	$ 1.63		$ 1.32		$ 1.04
Diluted EPS (USD per share)		$ 0.41	$ 0.42	$ 0.39	$ 0.39	$ 0.36	$ 0.35	$ 0.30	$ 0.29	$ 1.61		$ 1.31		$ 1.03
Antidilutive stock options and restricted stock units excluded from the calculation of diluted EPS (n shares)										15		26		23

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

[2] Antidilutive stock options and restricted stock units excluded from the calculation of diluted EPS totaled 15 million, 26 million, and 23 million shares in 2017, 2016, and 2015, respectively.