Derivative Instruments and Hedging Activities (Interest and Amortization on Derivative Instruments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Interest and dividends on investment securities and other earning assets	$ 240	$ 176
Interest on long-term debt	419	411
Earnings on cash flow hedges to be recognized within twelve months	7
Designated as hedging instrument | Interest and fees on finance receivables and loans | Fair value hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	0
Designated as hedging instrument | Interest and fees on finance receivables and loans | Amortization and interest | Fair value hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	2	(11)
Designated as hedging instrument | Interest and fees on finance receivables and loans | Amortization and interest | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedging relationships	0	0
Designated as hedging instrument | Interest and dividends on investment securities and other earning assets | Fair value hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	0
Designated as hedging instrument | Interest and dividends on investment securities and other earning assets | Amortization and interest | Fair value hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	(1)
Designated as hedging instrument | Interest and dividends on investment securities and other earning assets | Amortization and interest | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedging relationships	0	0
Designated as hedging instrument | Interest on long-term debt | Fair value hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	1
Designated as hedging instrument | Interest on long-term debt | Amortization and interest | Fair value hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	19
Designated as hedging instrument | Interest on long-term debt | Amortization and interest | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedging relationships	0	1
Designated as hedging instrument | Unsecured debt | Interest and fees on finance receivables and loans | Amortization and interest | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	0	0
Gain (loss) on interest for qualifying hedge	0	0
Designated as hedging instrument | Unsecured debt | Interest and dividends on investment securities and other earning assets | Amortization and interest | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	0	0
Gain (loss) on interest for qualifying hedge	0	0
Designated as hedging instrument | Unsecured debt | Interest on long-term debt | Amortization and interest | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	6	15
Gain (loss) on interest for qualifying hedge	0	3
Designated as hedging instrument | Federal Home Loan Bank certificates and obligations | Interest and fees on finance receivables and loans | Amortization and interest | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	0	0
Gain (loss) on interest for qualifying hedge	0	0
Designated as hedging instrument | Federal Home Loan Bank certificates and obligations | Interest and dividends on investment securities and other earning assets | Amortization and interest | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	0	0
Gain (loss) on interest for qualifying hedge	0	0
Designated as hedging instrument | Federal Home Loan Bank certificates and obligations | Interest on long-term debt | Amortization and interest | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	(6)	(1)
Gain (loss) on interest for qualifying hedge	0	2
Designated as hedging instrument | Available-for-sale securities | Interest and fees on finance receivables and loans | Amortization and interest | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	0	0
Designated as hedging instrument | Available-for-sale securities | Interest and dividends on investment securities and other earning assets | Amortization and interest | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	0	(1)
Designated as hedging instrument | Available-for-sale securities | Interest on long-term debt | Amortization and interest | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	0	0
Designated as hedging instrument | Fixed-rate automotive loans | Interest and fees on finance receivables and loans | Amortization and interest | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	(4)	(4)
Gain (loss) on interest for qualifying hedge	6	(7)
Designated as hedging instrument | Fixed-rate automotive loans | Interest and dividends on investment securities and other earning assets | Amortization and interest | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	0	0
Gain (loss) on interest for qualifying hedge	0	0
Designated as hedging instrument | Fixed-rate automotive loans | Interest on long-term debt | Amortization and interest | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on amortization of deferred basis adjustments	0	0
Gain (loss) on interest for qualifying hedge	0	0
Variable-rate borrowings | Designated as hedging instrument | Interest and fees on finance receivables and loans | Cash flow hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedging relationships	0	0
Variable-rate borrowings | Designated as hedging instrument | Interest and fees on finance receivables and loans | Amortization and interest | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	0	0
Variable-rate borrowings | Designated as hedging instrument | Interest and dividends on investment securities and other earning assets | Cash flow hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedging relationships	0	0
Variable-rate borrowings | Designated as hedging instrument | Interest and dividends on investment securities and other earning assets | Amortization and interest | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	0	0
Variable-rate borrowings | Designated as hedging instrument | Interest on long-term debt | Cash flow hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedging relationships	4	0
Variable-rate borrowings | Designated as hedging instrument | Interest on long-term debt | Amortization and interest | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	$ 0	$ 1