Credit Quality and the Allowance for Loan and Lease Losses - Summary of Transactions in the ALLL by Portfolio Segment (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2024		Mar. 31, 2023	Dec. 31, 2023		Dec. 31, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period	$ 2,322	[1]	$ 2,194	$ 2,194
Losses charged off	(146)		(110)
Recoveries of losses previously charged off	36		32
Provision for (benefit from) loan and lease losses	106		148
Balance, end of period	2,318	[1]	2,215	$ 2,322	[1]	$ 2,194
Accounting Standards Update [Extensible Enumeration]				Accounting Standard Update 2023-02 [Member]		Accounting Standards Update 2022-02 [Member]
Other Consumer Loans, Point of Sale
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Losses charged off	(8)		(9)
Recoveries of losses previously charged off	8		9
Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period			(49)	$ (49)
Balance, end of period						$ (49)
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period	1,130		1,127	1,127
Losses charged off	(40)		(33)
Recoveries of losses previously charged off	5		2
Provision for (benefit from) loan and lease losses	46		43
Balance, end of period	1,141		1,143	1,130		1,127
Commercial | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period			4	4
Balance, end of period						4
Residential Mortgage
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period	145		245	245
Losses charged off	0		(1)
Recoveries of losses previously charged off	0		1
Provision for (benefit from) loan and lease losses	(5)		(24)
Balance, end of period	140		185	145		245
Residential Mortgage | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period			(36)	(36)
Balance, end of period						(36)
Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period	1,047		822	822
Losses charged off	(106)		(76)
Recoveries of losses previously charged off	31		29
Provision for (benefit from) loan and lease losses	65		129
Balance, end of period	$ 1,037		887	1,047		822
Consumer | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period			$ (17)	$ (17)
Balance, end of period						$ (17)

[1] Includes $48 and $55 of other short-term investments, $1,420 and $1,573 of portfolio loans and leases, $(26) and $(28) of ALLL, $29 and $10 of other assets, $25 and $14 of other liabilities and $1,274 and $1,409 of long-term debt from consolidated VIEs that are included in their respective captions above at March 31, 2024 and December 31, 2023, respectively. For further information, refer to Note 11.