Derivative Instruments and Hedging Activities (Interest and Amortization on Derivative Instruments) (Details) - Designated as hedging instrument - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest And Amortization Income (Expense) Hedging Relationship	$ (6)	$ 2	$ (22)
Gain (loss) on cash flow hedging relationships	1	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)	(1)	0
Gain (loss) on cash flow hedging relationships	0	0	0
Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest And Amortization Income (Expense) Hedging Relationship	0	0	0
Gain (loss) on cash flow hedging relationships	(1)	3	0
Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest And Amortization Income (Expense) Hedging Relationship	2	48	102
Gain (loss) on cash flow hedging relationships	0	8	(1)
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
Gain (loss) on interest for qualifying hedge	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
Gain (loss) on interest for qualifying hedge	0	0	0
Unsecured debt | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	0	0	0
Gain (loss) on interest for qualifying hedge	0	0	0
Unsecured debt | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	25	51	77
Gain (loss) on interest for qualifying hedge	0	8	24
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
Gain (loss) on interest for qualifying hedge	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
Gain (loss) on interest for qualifying hedge	0	0	0
Federal Home Loan Bank certificates and obligations | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	0	0	0
Gain (loss) on interest for qualifying hedge	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	(23)	(17)	(2)
Gain (loss) on interest for qualifying hedge	0	6	3
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
Gain (loss) on interest for qualifying hedge	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	(3)	0	0
Gain (loss) on interest for qualifying hedge	2	(1)	0
Available-for-sale securities | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	0	0	0
Gain (loss) on interest for qualifying hedge	0	0	0
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
Gain (loss) on interest for qualifying hedge	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	(28)	(14)	(21)
Gain (loss) on interest for qualifying hedge	22	16	(1)
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
Gain (loss) on interest for qualifying hedge	0	0	0
Fixed-rate automotive loans | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	0	0	0
Gain (loss) on interest for qualifying hedge	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
Gain (loss) on interest for qualifying hedge	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
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
Variable-rate borrowings | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	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	8	(1)
Deposit liabilities | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	0	0	0
Deposit liabilities | 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
Deposit liabilities | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	(1)	3	0
Deposit liabilities | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	0	0	0
Variable-rate commercial borrowings | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	1	0	0
Variable-rate commercial 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
Variable-rate commercial borrowings | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	0	0	0
Variable-rate commercial borrowings | Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	$ 0	$ 0	$ 0