Allowance for Credit Losses - Rollforward of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 821	$ 737	$ 773	$ 685	$ 685
Charge-offs	(33)	(104)	(128)	(159)
Recoveries	9	9	30	25
Provision for credit losses and remaining unfunded commitments	26	118	148	215
Ending balance	823	760	823	760	773	$ 685
Off-Balance-Sheet, Credit Loss, Liability [Roll Forward]
Beginning balance	66	90	83	87	87
Provision for remaining unfunded commitments	9	(8)	(8)	(5)
Ending balance	75	82	75	82	83	$ 87
Allowance for credit losses	898	842	898	842
Loans and leases	62,445		62,445		$ 61,292
Accounting Standards Update [Extensible List]					Accounting Standards Update 2023-02	Accounting Standards Update 2022-02
Accounting Standards Update 2022-02
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				(6)	$ (6)
Ending balance						$ (6)
Commercial: | Commercial, financial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	344	326	339	308	308
Charge-offs	(12)	(91)	(64)	(124)
Recoveries	4	5	19	12
Provision for credit losses and remaining unfunded commitments	16	95	58	138
Ending balance	352	335	352	335	339	308
Off-Balance-Sheet, Credit Loss, Liability [Roll Forward]
Beginning balance	44	55	49	55	55
Provision for remaining unfunded commitments	11	(6)	6	(6)
Ending balance	55	49	55	49	49	55
Allowance for credit losses	407	384	407	384
Loans and leases	33,092		33,092		32,633
Commercial: | Commercial, financial and industrial | Single Company
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs				(72)
Commercial: | Commercial, financial and industrial | Paycheck Protection Plan
Off-Balance-Sheet, Credit Loss, Liability [Roll Forward]
Loans and leases	15	35	15	35	29
Commercial: | Commercial, financial and industrial | Accounting Standards Update 2022-02
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				1	1
Ending balance						1
Commercial: | Commercial Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	221	159	172	146	146
Charge-offs	(15)	(5)	(47)	(15)
Recoveries	1	1	1	2
Provision for credit losses and remaining unfunded commitments	11	14	92	36
Ending balance	218	169	218	169	172	146
Off-Balance-Sheet, Credit Loss, Liability [Roll Forward]
Beginning balance	10	24	22	22	22
Provision for remaining unfunded commitments	(2)	(3)	(14)	(1)
Ending balance	8	21	8	21	22	22
Allowance for credit losses	226	190	226	190
Loans and leases	14,705		14,705		14,216
Commercial: | Commercial Real Estate | Accounting Standards Update 2022-02
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				0	0
Ending balance						0
Commercial: | Credit Card and Other
Off-Balance-Sheet, Credit Loss, Liability [Roll Forward]
Loans and leases	184		184		180
Consumer:
Off-Balance-Sheet, Credit Loss, Liability [Roll Forward]
Loans and leases	13,961		13,961		13,650
Consumer: | Consumer Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	231	221	233	200	200
Charge-offs	(1)	(1)	(2)	(3)
Recoveries	3	2	6	7
Provision for credit losses and remaining unfunded commitments	(3)	6	(7)	31
Ending balance	230	228	230	228	233	200
Off-Balance-Sheet, Credit Loss, Liability [Roll Forward]
Beginning balance	12	11	12	10	10
Provision for remaining unfunded commitments	0	1	0	2
Ending balance	12	12	12	12	12	10
Allowance for credit losses	242	240	242	240
Loans and leases	13,961		13,961		13,650
Consumer: | Consumer Real Estate | Accounting Standards Update 2022-02
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				(7)	(7)
Ending balance						(7)
Consumer: | Credit Card and Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	25	31	29	31	31
Charge-offs	(5)	(7)	(15)	(17)
Recoveries	1	1	4	4
Provision for credit losses and remaining unfunded commitments	2	3	5	10
Ending balance	23	28	23	28	29	31
Off-Balance-Sheet, Credit Loss, Liability [Roll Forward]
Beginning balance	0	0	0	0	0
Provision for remaining unfunded commitments	0	0	0	0
Ending balance	0	0	0	0	0	0
Allowance for credit losses	23	$ 28	23	28
Loans and leases	$ 687		$ 687		793
Consumer: | Credit Card and Other | Accounting Standards Update 2022-02
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				$ 0	$ 0
Ending balance						$ 0