Allowance for Credit Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	$ 200	$ 180	$ 189
Charge-offs	(156)	(59)	(46)
Recoveries	36	32	30
Ending Balance	963	200	180
Loans, net of unearned income	58,232	31,061
Funded Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Provision for credit losses	489	47	7
Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	6	7	5
Provision for credit losses	14	(1)	2
Initial reserve on loans acquired	41
Ending Balance	85	6	7
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Loans, net of unearned income	33,104
Commercial | C&I
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	123	99	98
Charge-offs	(129)	(34)	(15)
Recoveries	9	7	4
Ending Balance	453	123	99
Commercial | C&I | PCD Loans
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs	(237)
Commercial | C&I | Non-PCD Loans
Allowance for Loan and Lease Losses [Roll Forward]
Provision for credit losses	147
Commercial | C&I | Paycheck Protection Plan
Allowance for Loan and Lease Losses [Roll Forward]
Loans, net of unearned income	4,100
Commercial | C&I | Funded Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Provision for credit losses	293	51	12
Commercial | C&I | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	4	4	3
Provision for credit losses	32	0	1
Initial reserve on loans acquired	12
Ending Balance	65	4	4
Commercial | Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	36	31	28
Charge-offs	(5)	(1)	(1)
Recoveries	4	1	1
Ending Balance	242	36	31
Commercial | Commercial real estate | Funded Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Provision for credit losses	114	5	3
Commercial | Commercial real estate | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	2	3	2
Provision for credit losses	(19)	(1)	1
Initial reserve on loans acquired	26
Ending Balance	10	2	3
Consumer | Consumer real estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	28	37	53
Charge-offs	(8)	(8)	(10)
Recoveries	18	20	21
Ending Balance	242	28	37
Loans, net of unearned income	11,725
Consumer | Consumer real estate | Funded Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Provision for credit losses	67	(21)	(27)
Consumer | Consumer real estate | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	0	0	0
Provision for credit losses	1	0	0
Initial reserve on loans acquired	3
Ending Balance	10	0	0
Consumer | Credit card and other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	13	13	10
Charge-offs	(14)	(16)	(20)
Recoveries	5	4	4
Ending Balance	26	13	13
Loans, net of unearned income	1,128
Consumer | Credit card and other | Funded Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Provision for credit losses	15	12	19
Consumer | Credit card and other | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	0	0	0
Provision for credit losses	0	0	0
Initial reserve on loans acquired	0
Ending Balance	0	0	$ 0
Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	107
Ending Balance		107
Adjustment | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	24
Ending Balance		24
Adjustment | Commercial | C&I
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	19
Ending Balance		19
Adjustment | Commercial | C&I | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	17
Ending Balance		17
Adjustment | Commercial | Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	(7)
Ending Balance		(7)
Adjustment | Commercial | Commercial real estate | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	1
Ending Balance		1
Adjustment | Consumer | Consumer real estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	93
Ending Balance		93
Adjustment | Consumer | Consumer real estate | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	6
Ending Balance		6
Adjustment | Consumer | Credit card and other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	2
Ending Balance		2
Adjustment | Consumer | Credit card and other | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	0
Ending Balance		0
Adjusted Balance
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	307
Ending Balance		307
Adjusted Balance | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	30
Ending Balance		30
Adjusted Balance | Commercial | C&I
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	142
Ending Balance		142
Adjusted Balance | Commercial | C&I | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	21
Ending Balance		21
Adjusted Balance | Commercial | Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	29
Ending Balance		29
Adjusted Balance | Commercial | Commercial real estate | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	3
Ending Balance		3
Adjusted Balance | Consumer | Consumer real estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	121
Ending Balance		121
Adjusted Balance | Consumer | Consumer real estate | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	6
Ending Balance		6
Adjusted Balance | Consumer | Credit card and other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	15
Ending Balance		15
Adjusted Balance | Consumer | Credit card and other | Unfunded Commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning Balance	0
Ending Balance		$ 0
Financial Asset Acquired with Credit Deterioration
Allowance for Loan and Lease Losses [Roll Forward]
Initial allowance on loans purchased with credit deterioration	287
Additions in allowance for credit loss	287
Financial Asset Acquired with Credit Deterioration | Commercial | C&I
Allowance for Loan and Lease Losses [Roll Forward]
Initial allowance on loans purchased with credit deterioration	138
Financial Asset Acquired with Credit Deterioration | Commercial | Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Initial allowance on loans purchased with credit deterioration	100
Financial Asset Acquired with Credit Deterioration | Consumer | Consumer real estate
Allowance for Loan and Lease Losses [Roll Forward]
Initial allowance on loans purchased with credit deterioration	44
Financial Asset Acquired with Credit Deterioration | Consumer | Credit card and other
Allowance for Loan and Lease Losses [Roll Forward]
Initial allowance on loans purchased with credit deterioration	$ 5