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		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	$ 16	$ 62	$ 54	$ 218
Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	21	56	62	164
Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	2	3	7	9
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	2	2	7	7
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	1	0	2
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	6	13	17
Gain on interest for qualifying hedge	17	50	49	147
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	(1)	3	3	12
Gain on interest for qualifying hedge	17	59	51	206
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