Parent Company Financial Information - Statements of Cash Flows (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net income attributable to controlling interest		$ 845	$ 441	$ 545
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation, amortization, and other		46	65	59
Deferred income tax expense (benefit)		(18)	14	104
Stock-based compensation expense		32	22	23
Total adjustments		(685)	378	(323)
Net cash provided by (used in) operating activities		172	830	234
Securities:
Proceeds from sales and prepayments of securities		629	192	21
Purchases of securities
Net cash provided by (used in) investing activities		(4,967)	(2,390)	480
Preferred stock:
Proceeds from issuance of preferred stock		144	0	0
Cash dividends paid - preferred stock		(17)	(6)	(6)
Common stock:
Stock options exercised		7	9	5
Cash dividends paid		(222)	(171)	(139)
Repurchase of shares	[1]	(4)	(134)	(105)
Proceeds from issuance of term borrowings		1,249	0	0
Term borrowings:
Repayment of term borrowings		(1,570)	(406)	(69)
Net cash provided by (used in) financing activities		5,176	1,422	(761)
Net increase (decrease) in cash and cash equivalents		381	(138)	(47)
Cash and cash equivalents at beginning of period		1,267	1,405	1,452
Cash and cash equivalents at end of period		1,648	1,267	1,405
Total interest paid		261	411	308
Income taxes received from subsidiaries		105	71	43
Parent Company
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net income attributable to controlling interest		845	441	545
Less undistributed net income of subsidiaries		740	158	170
Income before undistributed net income of subsidiaries		105	283	375
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation, amortization, and other		0	(1)	0
(Gain) loss on derivative transactions		4	0	0
Deferred income tax expense (benefit)		5	4	3
Stock-based compensation expense		32	22	23
Other operating activities, net		21	28	7
Total adjustments		62	53	33
Net cash provided by (used in) operating activities		167	336	408
Securities:
Proceeds from sales and prepayments of securities		0	1	0
Purchases of securities
Purchases of securities		(5)	0	0
Sales (purchases) of premises and equipment		(2)	0	2
Cash received (paid for) business combination, net		103	0	(40)
Net cash provided by (used in) investing activities		96	1	(38)
Preferred stock:
Proceeds from issuance of preferred stock		144	0	0
Cash dividends paid - preferred stock		(17)	(6)	(6)
Common stock:
Stock options exercised		7	9	4
Cash dividends paid		(222)	(171)	(139)
Repurchase of shares		(4)	(134)	(105)
Proceeds from issuance of term borrowings		795	0	0
Term borrowings:
Repayment of term borrowings		(500)	0	(45)
Other financing activities, net		(8)	0	0
Net cash provided by (used in) financing activities		195	(302)	(291)
Net increase (decrease) in cash and cash equivalents		458	35	79
Cash and cash equivalents at beginning of period		369	334	255
Cash and cash equivalents at end of period		827	369	334
Total interest paid		33	29	29
Income taxes received from subsidiaries		$ 33	$ 43	$ 49

[1] 2019 and 2018 include $130 million and $99 million, respectively, repurchased under share repurchase programs.