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, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 456	$ 460	$ 470	$ 477	$ 481	$ 463	$ 431	$ 434	$ 1,863	$ 1,809	$ 1,853
Interest expense	32	33	35	35	37	40	40	39	135	156	207
Net interest income	424	427	435	442	444	423	391	395	1,728	1,653	1,646
Provision for credit losses	16	22	19	22	18	35	45	46	79	144	478
Net securities gains (losses)	1	0	6	5	(4)	12	4	2	12	14	3
Noninterest income excluding net securities gains (losses)	203	197	205	201	186	189	198	205
Noninterest expenses	427	449	433	448	479	463	411	418	1,757	1,771	1,642
Provision for income taxes	55	36	50	48	33	28	41	35	189	137	55
Net income	130	117	144	130	96	98	96	103	521	393	277
Income allocated to participating securities	2	1	2	1	1	1	1	1	6	4	1
Net income attributable to common shares	128	116	142	129	95	97	95	102	515	389	153
Basic earnings per common share	$ 0.68	$ 0.61	$ 0.73	$ 0.66	$ 0.48	$ 0.51	$ 0.54	$ 0.58	$ 2.68	$ 2.11	$ 0.90
Diluted earnings per common share	$ 0.68	$ 0.61	$ 0.73	$ 0.66	$ 0.48	$ 0.51	$ 0.53	$ 0.57	$ 2.67	$ 2.09	$ 0.88
Comprehensive income (loss)	$ (30)	$ 165	$ 169	$ 160	$ (30)	$ 176	$ 170	$ 110	$ 464	$ 426	$ 224