Derivative Instruments and Hedging Activities (Interest and Amortization on Derivative Instruments) (Details) - Designated as hedging instrument - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest And Amortization Income (Expense) Hedging Relationship	$ (170)	$ (6)	$ 2
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	1	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	(13)	(1)	(1)
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	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 Hedge Ineffectiveness, Net	0	(1)	3
Interest on long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest And Amortization Income (Expense) Hedging Relationship	(10)	2	48
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	0	0	8
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	12	25	51
Gain (loss) on interest for qualifying hedge	0	0	8
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	(22)	(23)	(17)
Gain (loss) on interest for qualifying hedge	0	0	6
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	(7)	(3)	0
Gain (loss) on interest for qualifying hedge	(6)	2	(1)
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	(49)	(28)	(14)
Gain (loss) on interest for qualifying hedge	(121)	22	16
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	0	8
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	0	(1)	3
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 loans | Interest and fees on finance receivables and loans
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	1	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	0
Variable-rate commercial loans | Interest on deposits
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	0	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	$ 0