Derivative Instruments and Hedging Activities (Derivative Instruments Designated as Fair Value Hedges, Gain (Loss)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Interest and fees on finance receivables and loans	$ 6,688	$ 5,819	$ 5,162
Interest and dividends on investment securities and other earning assets	788	599	418
Interest on deposits	1,735	1,077	830
Interest on long-term debt	1,753	1,653	1,742
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	(2)	(2)
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	0
Designated as hedging instrument | Interest on deposits | Fair value hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	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	1	5	(5)
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
Change in unrealized gain (loss) on fair value hedging instruments	0	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	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0	0
Designated as hedging instrument | Unsecured debt | Interest on deposits | 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
Change in unrealized gain (loss) on fair value hedging instruments	0	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	62	8	(78)
Change in unrealized gain (loss) on fair value hedging instruments	(61)	(3)	73
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
Change in unrealized gain (loss) on fair value hedging instruments	0	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	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0	0
Designated as hedging instrument | Federal Home Loan Bank certificates and obligations | Interest on deposits | 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
Change in unrealized gain (loss) on fair value hedging instruments	0	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	47	22	8
Change in unrealized gain (loss) on fair value hedging instruments	(47)	(22)	(8)
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
Change in unrealized gain (loss) on fair value hedging instruments	0	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	(3)	(1)	0
Change in unrealized gain (loss) on fair value hedging instruments	3	1	0
Designated as hedging instrument | Available-for-sale securities | Interest on deposits | 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
Change in unrealized gain (loss) on fair value hedging instruments	0	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	0
Change in unrealized gain (loss) on fair value hedging instruments	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	19	(3)	0
Change in unrealized gain (loss) on fair value hedging instruments	(19)	1	(2)
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	0
Change in unrealized gain (loss) on fair value hedging instruments	0	0	0
Designated as hedging instrument | Fixed-rate automotive loans | Interest on deposits | 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
Change in unrealized gain (loss) on fair value hedging instruments	0	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	0
Change in unrealized gain (loss) on fair value hedging instruments	0	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]
Gain (loss) on cash flow hedges, reclassified to earnings, net	0	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]
Gain (loss) on cash flow hedges, reclassified to earnings, net	0	0	0
Deposit liabilities | Designated as hedging instrument | Interest on deposits | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedges, reclassified to earnings, net	1	0	0
Deposit liabilities | Designated as hedging instrument | Interest on long-term debt | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedges, reclassified to earnings, net	0	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	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]
Gain (loss) on cash flow hedges, reclassified to earnings, net	0	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	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]
Gain (loss) on cash flow hedges, reclassified to earnings, net	0	0	0
Variable-rate borrowings | Designated as hedging instrument | Interest on deposits | Cash flow hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedging relationships	1	0	0
Variable-rate borrowings | Designated as hedging instrument | Interest on deposits | Cash flow hedging | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on cash flow hedges, reclassified to earnings, net	0	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	1	0	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]
Gain (loss) on cash flow hedges, reclassified to earnings, net	$ 1	$ 0	$ 0