Derivative Instruments and Hedging Activities (Derivative Instruments Designated as Fair Value Hedges, Gain (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Interest and fees on finance receivables and loans	$ 1,807	$ 1,543
Interest and dividends on investment securities and other earning assets	240	176
Interest on deposits	592	351
Interest on long-term debt	419	411
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 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 on deposits [Member] | Fair value hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	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 | 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
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Unsecured debt | Interest and dividends on investment securities and other earning assets | 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
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Unsecured debt | interest on deposits [Member] | 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
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Unsecured debt | Interest on 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	36
Change in unrealized gain (loss) on fair value hedging instruments	0	(35)
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
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Federal Home Loan Bank certificates and obligations | Interest and dividends on investment securities and other earning assets | 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
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Federal Home Loan Bank certificates and obligations | interest on deposits [Member] | 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
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Federal Home Loan Bank certificates and obligations | Interest on 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	33
Change in unrealized gain (loss) on fair value hedging instruments	0	(33)
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
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Available-for-sale securities | Interest and dividends on investment securities and other earning assets | 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	(3)
Change in unrealized gain (loss) on fair value hedging instruments	(10)	3
Designated as hedging instrument | Available-for-sale securities | interest on deposits [Member] | 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
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Available-for-sale securities | Interest on 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
Change in unrealized gain (loss) on fair value hedging instruments	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	43	(45)
Change in unrealized gain (loss) on fair value hedging instruments	(43)	45
Designated as hedging instrument | Fixed-rate automotive loans | Interest and dividends on investment securities and other earning assets | 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
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Fixed-rate automotive loans | interest on deposits [Member] | 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
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Designated as hedging instrument | Fixed-rate automotive loans | Interest on 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
Change in unrealized gain (loss) on fair value hedging instruments	0	0
Deposit liabilities | Designated as hedging instrument | Interest and fees on finance receivables and loans | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0
Deposit liabilities | Designated as hedging instrument | Interest and dividends on investment securities and other earning assets | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0
Deposit liabilities | Designated as hedging instrument | interest on deposits [Member] | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	1	0
Deposit liabilities | Designated as hedging instrument | Interest on long-term debt | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	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 | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	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 | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0
Variable-rate borrowings | Designated as hedging instrument | interest on deposits [Member] | Cash flow hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedging relationships	1	0
Variable-rate borrowings | Designated as hedging instrument | interest on deposits [Member] | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	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 | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	$ 4	$ 0