Allowance for Credit Losses - Rollforward of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2023
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 773
Charge-offs	(46)	$ (22)
Ending balance	787	715
Loans and leases	61,753				$ 61,292
Accounting Standards Update [Extensible List]			Accounting Standards Update 2023-02	Accounting Standards Update 2022-02
Funded And Unfunded Loan Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Ending balance	866	800
Funded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	773	685
Charge-offs	(46)	(22)
Recoveries	6	6
Provision for loan and lease losses	54	52
Ending balance			$ 685
Accounting Standards Update [Extensible List]			Accounting Standards Update 2022-02
Funded commitment | Accounting Standards Update 2022-02
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		(6)
Ending balance			$ (6)
Unfunded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	83	87
Provision for loan and lease losses	(4)	(2)
Ending balance	79	85	87
Commercial: | Commercial, financial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs	(28)	(14)
Ending balance	348	325
Loans and leases	32,911				32,633
Commercial: | Commercial, financial and industrial | Paycheck Protection Plan
Allowance for Loan and Lease Losses [Roll Forward]
Loans and leases	23	53			29
Commercial: | Commercial, financial and industrial | Funded And Unfunded Loan Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Ending balance	397	378
Commercial: | Commercial, financial and industrial | Funded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	339	308
Charge-offs	(28)	(14)
Recoveries	3	3
Provision for loan and lease losses	34	27
Ending balance			308
Commercial: | Commercial, financial and industrial | Funded commitment | Accounting Standards Update 2022-02
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		1
Ending balance			1
Commercial: | Commercial, financial and industrial | Unfunded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	49	55
Provision for loan and lease losses	0	(2)
Ending balance	49	53	55
Commercial: | Commercial Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs	(12)	(2)
Ending balance	181	150
Loans and leases	14,426				14,216
Commercial: | Commercial Real Estate | Funded And Unfunded Loan Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Ending balance	199	171
Commercial: | Commercial Real Estate | Funded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	172	146
Charge-offs	(12)	(2)
Recoveries	0	0
Provision for loan and lease losses	21	6
Ending balance			146
Commercial: | Commercial Real Estate | Funded commitment | Accounting Standards Update 2022-02
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		0
Ending balance			0
Commercial: | Commercial Real Estate | Unfunded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	22	22
Provision for loan and lease losses	(4)	(1)
Ending balance	18	21	22
Commercial: | Credit Card and Other
Allowance for Loan and Lease Losses [Roll Forward]
Loans and leases	181				180
Consumer:
Allowance for Loan and Lease Losses [Roll Forward]
Loans and leases	13,645				13,650
Consumer: | Consumer Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs	0	(1)
Ending balance	231	209
Loans and leases	13,645				13,650
Consumer: | Consumer Real Estate | Funded And Unfunded Loan Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Ending balance	243	220
Consumer: | Consumer Real Estate | Funded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	233	200
Charge-offs	0	(1)
Recoveries	1	2
Provision for loan and lease losses	(3)	15
Ending balance			200
Consumer: | Consumer Real Estate | Funded commitment | Accounting Standards Update 2022-02
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		(7)
Ending balance			(7)
Consumer: | Consumer Real Estate | Unfunded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	12	10
Provision for loan and lease losses	0	1
Ending balance	12	11	10
Consumer: | Credit Card and Other
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs	(6)	(5)
Ending balance	27	31
Loans and leases	771				$ 793
Consumer: | Credit Card and Other | Funded And Unfunded Loan Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Ending balance	27	31
Consumer: | Credit Card and Other | Funded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	29	31
Charge-offs	(6)	(5)
Recoveries	2	1
Provision for loan and lease losses	2	4
Ending balance			31
Consumer: | Credit Card and Other | Funded commitment | Accounting Standards Update 2022-02
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		0
Ending balance			0
Consumer: | Credit Card and Other | Unfunded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	0
Provision for loan and lease losses	0	0
Ending balance	$ 0	$ 0	$ 0