Derivative Instruments and Hedging Activities (Derivative Instruments Designated as Fair Value Hedges, Gain (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Interest and fees on finance receivables and loans	$ 1,708	$ 1,486	$ 4,898	$ 4,301
Interest and dividends on investment securities and other earning assets	198	157	562	437
Interest on long-term debt	451	416	1,296	1,257
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	0	(2)
Designated as hedging instrument | Interest and dividends income | Fair value hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	0	0	0
Designated as hedging instrument | Interest expense, long-term debt | Fair value hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	0	1	1
Designated as hedging instrument | Unsecured debt | Interest and fees on finance receivables and loans | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0	0	0
Designated as hedging instrument | Unsecured debt | Interest and dividends income | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0	0	0
Designated as hedging instrument | Unsecured debt | Interest expense, long-term debt | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	20	0	64	(23)
Change in unrealized gain (loss) on fair value hedging instruments	(20)	0	(63)	24
Designated as hedging instrument | Federal Home Loan Bank certificates and obligations | Interest and fees on finance receivables and loans | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0	0	0
Designated as hedging instrument | Federal Home Loan Bank certificates and obligations | Interest and dividends income | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0	0	0
Designated as hedging instrument | Federal Home Loan Bank certificates and obligations | Interest expense, long-term debt | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	10	5	53	5
Change in unrealized gain (loss) on fair value hedging instruments	(10)	(5)	(53)	(5)
Designated as hedging instrument | Available-for-sale securities | Interest and fees on finance receivables and loans | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0	0	0
Designated as hedging instrument | Available-for-sale securities | Interest and dividends income | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	(2)	(3)	(7)	(1)
Change in unrealized gain (loss) on fair value hedging instruments	2	3	7	1
Designated as hedging instrument | Available-for-sale securities | Interest expense, long-term debt | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0	0	0
Designated as hedging instrument | Fixed-rate automotive loans | Interest and fees on finance receivables and loans | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	(9)	0	(60)	(3)
Change in unrealized gain (loss) on fair value hedging instruments	9	0	60	1
Designated as hedging instrument | Fixed-rate automotive loans | Interest and dividends income | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0	0	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0	0	0
Designated as hedging instrument | Fixed-rate automotive loans | Interest expense, long-term debt | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	0	0	0
Change in unrealized gain (loss) on fair value hedging instruments	$ 0	$ 0	$ 0	$ 0