Derivative Instruments and Hedging Activities (Derivative Instruments Designated as Fair Value Hedges, Gain (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Interest and fees on finance receivables and loans	$ 1,860	$ 1,647	$ 3,667	$ 3,190
Interest and dividends on investment securities and other earning assets	244	188	484	364
Interest on deposits	651	399	1,243	750
Interest on long-term debt	407	434	826	845
Earnings on cash flow hedges to be recognized within twelve months			0
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	98	(6)	141	(51)
Change in unrealized gain (loss) on fair value hedging instruments	(98)	6	(141)	51
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
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	2	(2)	12	(5)
Change in unrealized gain (loss) on fair value hedging instruments	(2)	2	(12)	5
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
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
Derivative, Gain (Loss) on Derivative, Net			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
Change in unrealized gain (loss) on fair value hedging instruments			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
Change in unrealized gain (loss) on fair value hedging instruments			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
Change in unrealized gain (loss) on fair value hedging instruments			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
Change in unrealized gain (loss) on fair value hedging instruments			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			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
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	8	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	(19)	8	(19)	44
Change in unrealized gain (loss) on fair value hedging instruments	19	(8)	19	(43)
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	43
Change in unrealized gain (loss) on fair value hedging instruments	0	(10)	0	(43)
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
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	$ 8	$ 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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