Earnings Per Common Share (EPS under Basic and Diluted Computations) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Earnings Per Common Share [Abstract]
Net income	$ 350	$ 321	$ 324	$ 326	$ 319	$ 290	$ 256	$ 206	$ 1,321		$ 1,071		$ 928
Preferred stock dividends									(60)		(61)		(45)
Net income available to common stockholders	$ 329	$ 312	$ 302	$ 318	$ 297	$ 282	$ 233	$ 198	$ 1,261		$ 1,010		$ 883
Weighted-average common shares outstanding — basic									1,303		1,285		1,274
Common stock equivalent shares related to stock incentive plans									12		8		1
Weighted-average common shares outstanding — diluted	1,320	1,316	1,313	1,311	1,304	1,296	1,288	1,282	1,315	[1]	1,293	[1]	1,275	[1]
Basic EPS	$ 0.25	$ 0.24	$ 0.23	$ 0.24	$ 0.23	$ 0.22	$ 0.18	$ 0.15	$ 0.96		$ 0.78		$ 0.69
Diluted EPS	$ 0.25	$ 0.24	$ 0.23	$ 0.24	$ 0.23	$ 0.22	$ 0.18	$ 0.15	$ 0.95		$ 0.78		$ 0.69
Antidilutive stock options and restricted stock awards excluded from the calculation of diluted EPS									24		34		74

[1] Antidilutive stock options and restricted stock awards excluded from the calculation of diluted EPS totaled 24 million, 34 million, and 74 million shares in 2014, 2013, and 2012, respectively.