Derivative Instruments and Hedging Activities (Interest and Amortization on Derivative Instruments) (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
Earnings on cash flow hedges to be recognized within twelve months	21		21
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 fees on finance receivables and loans | Amortization and interest | Fair value hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	4	(6)	(6)	(18)
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 hedge ineffectiveness, net	0	0	0	0
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 and dividends income | Amortization and interest | Fair value hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	0	(1)	0
Designated as hedging instrument | Interest and dividends income | Amortization and interest | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedge ineffectiveness, net	0	0	0	0
Designated as hedging instrument | Interest expense, deposits | Amortization and interest | 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, deposits | Amortization and interest | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedge ineffectiveness, net	2	0	2	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 | Interest expense, long-term debt | Amortization and interest | Fair value hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	12	26	43	77
Designated as hedging instrument | Interest expense, long-term debt | Amortization and interest | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedge ineffectiveness, net	3	0	6	0
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	0	0
Gain (loss) on interest for qualifying hedge	0	0	0	0
Designated as hedging instrument | Unsecured debt | Interest and dividends income | 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	0	0
Gain (loss) on interest for qualifying hedge	0	0	0	0
Designated as hedging instrument | Unsecured debt | Interest expense, deposits | 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	0	0
Gain (loss) on interest for qualifying hedge	0	0	0	0
Designated as hedging instrument | Unsecured debt | Interest expense, 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	13	19	42	59
Gain (loss) on interest for qualifying hedge	3	7	7	19
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	0	0
Gain (loss) on interest for qualifying hedge	0	0	0	0
Designated as hedging instrument | Federal Home Loan Bank certificates and obligations | Interest and dividends income | 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	0	0
Gain (loss) on interest for qualifying hedge	0	0	0	0
Designated as hedging instrument | Federal Home Loan Bank certificates and obligations | Interest expense, deposits | 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	0	0
Gain (loss) on interest for qualifying hedge	0	0	0	0
Designated as hedging instrument | Federal Home Loan Bank certificates and obligations | Interest expense, 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)	(12)	(2)
Gain (loss) on interest for qualifying hedge	2	1	6	1
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 income | Amortization and interest | Fair value hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge			(1)	0
Designated as hedging instrument | Available-for-sale securities | Interest expense, deposits | 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 expense, 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	(3)	(6)	(11)	(17)
Gain (loss) on interest for qualifying hedge	7	0	5	(1)
Designated as hedging instrument | Fixed-rate automotive loans | Interest and dividends income | 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	0	0
Gain (loss) on interest for qualifying hedge	0	0	0	0
Designated as hedging instrument | Fixed-rate automotive loans | Interest expense, deposits | 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	0	0
Gain (loss) on interest for qualifying hedge	0	0	0	0
Designated as hedging instrument | Fixed-rate automotive loans | Interest expense, 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	0	0
Gain (loss) on interest for qualifying hedge	0	0	0	0
Deposit Liabilities [Member] | 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	0	0
Deposit Liabilities [Member] | Designated as hedging instrument | Interest and dividends income | Amortization and interest | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	0	0	0	0
Deposit Liabilities [Member] | Designated as hedging instrument | Interest expense, deposits | Amortization and interest | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	0	0	0	0
Deposit Liabilities [Member] | Designated as hedging instrument | Interest expense, 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	3	0	6	0
Deposit Liabilities [Member] | 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	0	0
Deposit Liabilities [Member] | Designated as hedging instrument | Interest and dividends income | Amortization and interest | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	0	0	0	0
Deposit Liabilities [Member] | Designated as hedging instrument | Interest expense, deposits | Amortization and interest | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on interest for qualifying hedge	2	0	2	0
Deposit Liabilities [Member] | Designated as hedging instrument | Interest expense, 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	$ 0	$ 0	$ 0