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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Interest and fees on finance receivables and loans	$ 1,859	$ 1,708	$ 5,526	$ 4,898
Interest and dividends on investment securities and other earning assets	237	198	721	562
Interest on deposits	658	462	1,901	1,212
Interest on long-term debt	378	451	1,204	1,296
Earnings on cash flow hedges to be recognized within twelve months			5
Interest and fees on finance receivables and loans \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	0	0	0
Derivative, Gain (Loss) on Derivative, Net	0	0	0	0
Interest and fees on finance receivables and loans \| Unsecured debt \| Designated as hedging instrument
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
Interest and fees on finance receivables and loans \| Federal Home Loan Bank certificates and obligations \| Designated as hedging instrument
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
Interest and fees on finance receivables and loans \| Available-for-sale securities \| Designated as hedging instrument
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
Interest and fees on finance receivables and loans \| Fixed-rate automotive loans \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	32	(9)	173	(60)
Change in unrealized gain (loss) on fair value hedging instruments	(32)	9	(173)	60
Interest and fees on finance receivables and loans \| Deposit liabilities \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0	0	0
Interest and fees on finance receivables and loans \| Variable-rate borrowings \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0	0	0
Interest and dividends on investment securities and other earning assets \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	0	0	0
Derivative, Gain (Loss) on Derivative, Net	0	0	0	0
Interest and dividends on investment securities and other earning assets \| Unsecured debt \| Designated as hedging instrument
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
Interest and dividends on investment securities and other earning assets \| Federal Home Loan Bank certificates and obligations \| Designated as hedging instrument
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
Interest and dividends on investment securities and other earning assets \| Available-for-sale securities \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	17	(2)	29	(7)
Change in unrealized gain (loss) on fair value hedging instruments	(17)	2	(29)	7
Interest and dividends on investment securities and other earning assets \| Fixed-rate automotive loans \| Designated as hedging instrument
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
Interest and dividends on investment securities and other earning assets \| Deposit liabilities \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0	0	0
Interest and dividends on investment securities and other earning assets \| Variable-rate borrowings \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0	0	0
Interest on deposits \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	0	0	0
Derivative, Gain (Loss) on Derivative, Net	(2)	0	(1)	0
Interest on deposits \| Unsecured debt \| Designated as hedging instrument
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
Interest on deposits \| Federal Home Loan Bank certificates and obligations \| Designated as hedging instrument
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
Interest on deposits \| Available-for-sale securities \| Designated as hedging instrument
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
Interest on deposits \| Fixed-rate automotive loans \| Designated as hedging instrument
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
Interest on deposits \| Deposit liabilities \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	(2)	0	(1)	0
Interest on deposits \| Variable-rate borrowings \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0	0	0
Interest on long-term debt \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on fair value hedges	0	0	0	1
Derivative, Gain (Loss) on Derivative, Net	4	0	12	0
Interest on long-term debt \| Unsecured debt \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	(36)	20	(55)	64
Change in unrealized gain (loss) on fair value hedging instruments	36	(20)	55	(63)
Interest on long-term debt \| Federal Home Loan Bank certificates and obligations \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized gain (loss) on hedged item in fair value hedge	0	10	0	53
Change in unrealized gain (loss) on fair value hedging instruments	0	(10)	0	(53)
Interest on long-term debt \| Available-for-sale securities \| Designated as hedging instrument
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
Interest on long-term debt \| Fixed-rate automotive loans \| Designated as hedging instrument
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
Interest on long-term debt \| Deposit liabilities \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	0	0	0	0
Interest on long-term debt \| Variable-rate borrowings \| Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Interest rate cash flow hedge gain (loss) reclassified to earnings	$ 4	$ 0	$ 12	$ 0

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

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Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

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Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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Total amount of gain (loss) derived from fair value hedges recognized in earnings in the period.

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