Derivative Instruments and Hedging Activities (Interest and Amortization on Derivative Instruments) (Details) - Designated as hedging instrument - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest And Amortization Income (Expense) Hedging Relationship	$ (49)	$ 2	$ (122)	$ 6
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net			1	0
Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest And Amortization Income (Expense) Hedging Relationship	(4)	1	(9)	0
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net			0	0
Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest And Amortization Income (Expense) Hedging Relationship	(2)	0	(6)	1
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net			0	0
Unsecured debt | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) 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 (loss) on amortization of deferred basis adjustments	0	0	0	0
Unsecured debt | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	3	6	11	18
Federal Home Loan Bank certificates and obligations | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) 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 (loss) 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 (loss) on amortization of deferred basis adjustments	(5)	(6)	(17)	(17)
Available-for-sale securities | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	0	0	0	0
Gain (loss) on interest for qualifying hedge	0	0	0	0
Available-for-sale securities | Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	(2)	(1)	(5)	(2)
Gain (loss) on interest for qualifying hedge	(2)	2	(4)	2
Available-for-sale securities | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	0	0	0	0
Gain (loss) 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 (loss) on amortization of deferred basis adjustments	(12)	(8)	(38)	(21)
Gain (loss) on interest for qualifying hedge	(37)	10	(84)	27
Fixed-rate automotive loans | Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	0	0	0	0
Gain (loss) 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 (loss) on amortization of deferred basis adjustments	0	0	0	0
Gain (loss) on interest for qualifying hedge	$ 0	$ 0	0	0
Variable-rate commercial loans | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge			1	0
Variable-rate commercial loans | Interest and dividends on investment securities and other earning assets
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge			0	0
Variable-rate commercial loans | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge			$ 0	$ 0