Business Segment Information - Amounts of Consolidated Revenue, Expense, Tax and Assets by Segment (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Net interest income (expense)	$ 497		$ 305		$ 1,004	$ 608
Provision for credit losses	(115)		121		(160)	275
Noninterest income	285		206		583	381
Noninterest expense	498		320		1,042	623
Income (loss) before income taxes	399		70		705	91
Income tax expense (benefit)	88		13		159	18
Net income	311	$ 236	57	$ 16	546	73
Average assets	87,559		47,934		86,486	45,743
Regional Banking
Disaggregation of Revenue [Line Items]
Net interest income (expense)	443		211		869	405
Provision for credit losses	(88)		103		(120)	200
Noninterest income	108		69		208	142
Noninterest expense	278		164		551	338
Income (loss) before income taxes	361		13		646	9
Income tax expense (benefit)	85		1		151	(2)
Net income	276		12		495	11
Average assets	42,199		20,965		42,284	20,004
Specialty Banking
Disaggregation of Revenue [Line Items]
Net interest income (expense)	153		130		311	239
Provision for credit losses	(21)		18		(28)	72
Noninterest income	150		124		335	228
Noninterest expense	147		111		300	221
Income (loss) before income taxes	177		125		374	174
Income tax expense (benefit)	43		31		90	43
Net income	134		94		284	131
Average assets	20,055		18,041		20,775	17,466
Corporate
Disaggregation of Revenue [Line Items]
Net interest income (expense)	(99)		(36)		(176)	(36)
Provision for credit losses	(6)		0		(12)	3
Noninterest income	27		13		40	11
Noninterest expense	73		45		191	64
Income (loss) before income taxes	(139)		(68)		(315)	(92)
Income tax expense (benefit)	(40)		(19)		(82)	(23)
Net income	(99)		(49)		(233)	(69)
Average assets	$ 25,305		$ 8,928		23,427	$ 8,273
Asset impairment charges					$ 33