Derivative Instruments and Hedging Activities - Schedule of Interest and Amortization on Derivative Instruments (Details) - Total derivatives designated as accounting hedges - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	$ 256	$ 648	$ 147
Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	200	157	16
Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	11	11	3
Unsecured debt | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	0	0	0
Gain on interest for qualifying hedge	0	0	0
Unsecured debt | Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	0	0	0
Gain on interest for qualifying hedge	0	0	0
Unsecured debt | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	9	9	5
Gain on interest for qualifying hedge	0	0	1
Federal Home Loan Bank certificates and obligations | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	0	0	0
Federal Home Loan Bank certificates and obligations | Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	0	0	0
Federal Home Loan Bank certificates and obligations | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	2	2	(3)
Hedged available-for-sale securities | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	0	0	0
Gain on interest for qualifying hedge	0	0	0
Hedged available-for-sale securities | Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	23	23	17
Gain on interest for qualifying hedge	177	134	(1)
Hedged available-for-sale securities | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	0	0	0
Gain on interest for qualifying hedge	0	0	0
Fixed-rate automotive loans | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	15	32	18
Gain on interest for qualifying hedge	241	616	129
Fixed-rate automotive loans | Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	0	0	0
Gain on interest for qualifying hedge	0	0	0
Fixed-rate automotive loans | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	0	0	0
Gain on interest for qualifying hedge	$ 0	$ 0	$ 0