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		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	$ 17	$ 69	$ 38	$ 156
Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	19	54	41	108
Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	3	3	5	6
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	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	0
Unsecured debt | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	3	3	5	5
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	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	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	0	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	0	0
Gain on interest for qualifying hedge	0	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	4	5	9	11
Gain on interest for qualifying hedge	15	49	32	97
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	0
Gain on interest for qualifying hedge	0	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	2	4	4	9
Gain on interest for qualifying hedge	15	65	34	147
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	0
Gain on interest for qualifying hedge	0	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	0
Gain on interest for qualifying hedge	$ 0	$ 0	$ 0	$ 0