Derivative Instruments and Hedging Activities (Interest and Amortization on Derivative Instruments) (Details) - Designated as hedging instrument - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest And Amortization Income (Expense) Hedging Relationship	$ 2	$ 1	$ 4	$ (10)
Gain (loss) on cash flow hedging relationships	0	0	0	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	(1)	0	(1)	(1)
Gain (loss) on cash flow hedging relationships	0	0	0	0
Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest And Amortization Income (Expense) Hedging Relationship	1	12	1	31
Gain (loss) on cash flow hedging relationships	0	2	1	3
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
Gain (loss) on interest for qualifying hedge	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
Gain (loss) on interest for qualifying hedge	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	6	14	12	29
Gain (loss) on interest for qualifying hedge	0	1	0	4
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
Gain (loss) on interest for qualifying hedge	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
Gain (loss) on interest for qualifying hedge	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)	(5)	(11)	(6)
Gain (loss) on interest for qualifying hedge	0	2	0	4
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
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	(1)	0	(1)	0
Gain (loss) on interest for qualifying hedge			0	(1)
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
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	(9)	(4)	(13)	(8)
Gain (loss) on interest for qualifying hedge	11	5	17	(2)
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 borrowings | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	0	0	0	0
Variable-rate borrowings | 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	0	0
Variable-rate borrowings | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	$ 0	$ 2	$ 0	$ 3