Summary of Quarterly Financial Statements (Unaudited) Summary of Quarterly Financial Statements (Unaudited) (Summary of Quarterly Financial Statements) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 457	$ 448	$ 444	$ 435	$ 438	$ 436	$ 441	$ 435	$ 1,784	$ 1,750	$ 1,784
Interest expense	24	26	23	22	23	22	25	25	95	95	112
Net interest income	433	422	421	413	415	414	416	410	1,689	1,655	1,672
Provision for credit losses	60	26	47	14	2	5	11	9	147	27	46
Net securities (losses) gains	0	0	0	(2)	0	(1)	0	1	(2)	0	(1)
Noninterest income excluding net securities (losses) gains	270	264	261	257	225	216	220	207
Noninterest expenses	486	461	436	459	419	397	404	406	1,842	1,626	1,722
Provision for income taxes	41	63	64	61	70	73	70	64	229	277	245
Net Income	116	136	135	134	149	154	151	139	521	593	541
Less income allocated to participating securities	1	2	1	2	1	2	2	2	6	7	8
Net income attributable to common shares	$ 115	$ 134	$ 134	$ 132	$ 148	$ 152	$ 149	$ 137	$ 515	$ 586	$ 533
Basic earnings per common share	$ 0.65	$ 0.76	$ 0.76	$ 0.75	$ 0.83	$ 0.85	$ 0.83	$ 0.76	$ 2.93	$ 3.28	$ 2.92
Diluted earnings per common share	$ 0.64	$ 0.74	$ 0.73	$ 0.73	$ 0.80	$ 0.82	$ 0.80	$ 0.73	$ 2.84	$ 3.16	$ 2.85
Comprehensive income	$ 31	$ 187	$ 109	$ 176	$ 54	$ 141	$ 172	$ 205	$ 504	$ 572	$ 563