Summary of Quarterly Financial Statements (Unaudited) Summary of Quarterly Financial Statements (Unaudited) (Summary of Quarterly Financial Statements) (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
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 438	$ 436	$ 441	$ 435	$ 456	$ 439	$ 443	$ 446	$ 1,750	$ 1,784	$ 1,863
Interest expense	23	22	25	25	26	27	29	30	95	112	135
Net interest income	415	414	416	410	430	412	414	416	1,655	1,672	1,728
Provision for credit losses	2	5	11	9	9	8	13	16	27	46	79
Net securities (losses) gains	0	(1)	0	1	0	1	(2)	0	0	(1)	12
Noninterest income excluding net securities (losses) gains	225	216	220	207	219	227	224	213
Noninterest expenses	419	397	404	406	473	417	416	416	1,626	1,722	1,757
Provision for income taxes	70	73	70	64	50	68	64	63	277	245	241
Net Income	149	154	151	139	117	147	143	134	593	541	521
Less income allocated to participating securities	1	2	2	2	2	2	2	2	7	8	6
Net income attributable to common shares	148	152	149	137	115	145	141	132	586	533	515
Basic earnings per common share	$ 0.83	$ 0.85	$ 0.83	$ 0.76	$ 0.64	$ 0.80	$ 0.77	$ 0.71	$ 3.28	$ 2.92	$ 2.68
Diluted earnings per common share	$ 0.80	$ 0.82	$ 0.80	$ 0.73	$ 0.62	$ 0.78	$ 0.76	$ 0.70	$ 3.16	$ 2.85	$ 2.67
Comprehensive income	$ 54	$ 141	$ 172	$ 205	$ 267	$ 144	$ 15	$ 137	$ 572	$ 563	$ 464