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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 659	$ 711	$ 727	$ 710	$ 704	$ 675	$ 590	$ 650	$ 2,807	$ 2,619	$ 2,182
Interest expense	115	125	124	104	90	76	41	60	468	267	121
Net interest income	544	586	603	606	614	599	549	590	2,339	2,352	2,061
Provision for credit losses	8	35	44	(13)	16	0	12	(29)	74	(1)	74
Net securities losses	1	0	0	(8)	0	(20)	1	0	(7)	(19)	0
Noninterest income excluding net securities losses	265	256	250	246	250	254	243	248
Noninterest expenses	451	435	424	433	448	452	446	448	1,743	1,794	1,860
Provision for income taxes	82	80	87	85	90	63	54	93	334	300	491
Net Income	269	292	298	339	310	318	281	326	1,198	1,235	743
Less income allocated to participating securities	2	2	1	2	2	2	2	2	7	8	5
Net income attributable to common shares	$ 267	$ 290	$ 297	$ 337	$ 308	$ 316	$ 279	$ 324	$ 1,191	$ 1,227	$ 738
Basic earnings per common share	$ 1.87	$ 1.98	$ 1.95	$ 2.14	$ 1.91	$ 1.89	$ 1.62	$ 1.90	$ 7.95	$ 7.31	$ 4.23
Diluted earnings per common share	$ 1.85	$ 1.96	$ 1.94	$ 2.11	$ 1.88	$ 1.86	$ 1.59	$ 1.87	$ 7.87	$ 7.20	$ 4.14
Comprehensive income	$ 370	$ 338	$ 429	$ 435	$ 312	$ 296	$ 178	$ 290	$ 1,572	$ 1,076	$ 762