Derivative Instruments and Hedging Activities - Schedule of Interest and Amortization on Derivative Instruments (Details) - Total derivatives designated as accounting hedges - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	$ 21	$ 87
Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	22	54
Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	2	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
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
Unsecured debt | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	2	2
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
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
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	0	1
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
Gain on interest for qualifying hedge	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	5	6
Gain on interest for qualifying hedge	17	48
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
Gain on interest for qualifying hedge	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	2	5
Gain on interest for qualifying hedge	19	82
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
Gain on interest for qualifying hedge	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
Gain on interest for qualifying hedge	$ 0	$ 0