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, 2026	Mar. 31, 2025
Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	$ 5	$ 21
Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	4	22
Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Total gain on fair value hedging relationships	2	2
Unsecured \| Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	0	0
Unsecured \| 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 \| Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain on amortization of deferred basis adjustments	2	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
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	3	5
Gain on interest for qualifying hedge	1	17
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	0	2
Gain on interest for qualifying hedge	5	19
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