Parent Company FInancial Statements Parent Company Financial Statements (Statements of Cash Flows) (Details) - USD ($) $ 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
Condensed Financial Statements, Captions [Line Items]
Net income	$ 269	$ 292	$ 298	$ 339	$ 310	$ 318	$ 281	$ 326	$ 1,198	$ 1,235	$ 743
Depreciation and amortization									114	120	121
Net periodic defined benefit credit									(29)	(18)	(18)
Share-based compensation expense									39	48	39
Benefit for deferred income taxes									12	24	79
Other, net									(318)	184	39
Net cash provided by operating activities									1,090	1,616	1,085
Net change in premises and equipment									(86)	(90)	(69)
Net cash used in investing activities									(494)	(1,156)	(120)
Maturities of medium- and long-term debt									(350)	0	(500)
Repurchases of common stock									(1,394)	(1,338)	(560)
Cash dividends paid on common stock									(402)	(263)	(180)
Issuances of common stock under employee stock plans									18	52	118
Net cash provided by (used in) financing activities									661	(1,744)	(2,338)
Net (decrease) increase in cash and cash equivalents									1,257	(1,284)	(1,373)
Cash and cash equivalents at beginning of period				4,561			5,845		4,561	5,845	7,218
Cash and cash equivalents at end of period	5,818				4,561			5,845	5,818	4,561	5,845
Interest paid									462	261	122
Income taxes recovered									266	200	336
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net income									1,198	1,235	743
Undistributed earnings of subsidiaries, principally banks									6	(106)	113
Depreciation and amortization									1	1	1
Net periodic defined benefit credit									4	4	(2)
Share-based compensation expense									15	21	16
Benefit for deferred income taxes									(2)	(1)	(10)
Other, net									28	10	59
Net cash provided by operating activities									1,250	1,164	920
Maturities of medium- and long-term debt									(350)	0	0
Repurchases of common stock									(1,394)	(1,338)	(560)
Cash dividends paid on common stock									(402)	(263)	(180)
Issuances of common stock under employee stock plans									18	52	118
Net cash provided by (used in) financing activities									(1,578)	(699)	(622)
Net (decrease) increase in cash and cash equivalents									(328)	465	298
Cash and cash equivalents at beginning of period				$ 1,524			$ 1,059		1,524	1,059	761
Cash and cash equivalents at end of period	$ 1,196				$ 1,524			$ 1,059	1,196	1,524	1,059
Interest paid									55	11	12
Income taxes recovered									$ (226)	$ (155)	$ (331)