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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Interest and Fee Income, Loans and Leases	$ 1,647	$ 1,447	$ 3,190	$ 2,815
Interest and Dividend Income, Securities, Operating, Available-for-sale	188	146	364	280
Interest Expense, Long-term Debt	434	417	845	841
Designated as Hedging Instrument [Member] \| Interest and Fees on Finance Receivables and Loans [Member] \| Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	0	0	0	(2)
Designated as Hedging Instrument [Member] \| Interest and Dividends Income [Member] \| Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	0	0	0	0
Designated as Hedging Instrument [Member] \| Interest Expense [Member] \| Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	0	0	1	1
Designated as Hedging Instrument [Member] \| Unsecured Debt [Member] \| Interest and Fees on Finance Receivables and Loans [Member] \| Fair Value Hedging [Member] \| Interest Rate Contract [Member]
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
Designated as Hedging Instrument [Member] \| Unsecured Debt [Member] \| Interest and Dividends Income [Member] \| Fair Value Hedging [Member] \| Interest Rate Contract [Member]
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
Designated as Hedging Instrument [Member] \| Unsecured Debt [Member] \| Interest Expense [Member] \| Fair Value Hedging [Member] \| Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	8	(21)	44	(23)
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	(8)	21	(43)	24
Designated as Hedging Instrument [Member] \| Federal Home Loan Bank Certificates and Obligations (FHLB) [Member] \| Interest and Fees on Finance Receivables and Loans [Member] \| Fair Value Hedging [Member] \| Interest Rate Contract [Member]
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
Designated as Hedging Instrument [Member] \| Federal Home Loan Bank Certificates and Obligations (FHLB) [Member] \| Interest and Dividends Income [Member] \| Fair Value Hedging [Member] \| Interest Rate Contract [Member]
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
Designated as Hedging Instrument [Member] \| Federal Home Loan Bank Certificates and Obligations (FHLB) [Member] \| Interest Expense [Member] \| Fair Value Hedging [Member] \| Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	10	1	43	0
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	(10)	(1)	(43)	0
Designated as Hedging Instrument [Member] \| Available-for-sale Securities [Member] \| Interest and Fees on Finance Receivables and Loans [Member] \| Fair Value Hedging [Member] \| Interest Rate Contract [Member]
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
Designated as Hedging Instrument [Member] \| Available-for-sale Securities [Member] \| Interest and Dividends Income [Member] \| Fair Value Hedging [Member] \| Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	(2)	2	(5)	2
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	2	(2)	5	(2)
Designated as Hedging Instrument [Member] \| Available-for-sale Securities [Member] \| Interest Expense [Member] \| Fair Value Hedging [Member] \| Interest Rate Contract [Member]
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
Designated as Hedging Instrument [Member] \| Fixed-rate Automotive Loans [Member] \| Interest and Fees on Finance Receivables and Loans [Member] \| Fair Value Hedging [Member] \| Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	(6)	1	(51)	(3)
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	6	(1)	51	1
Designated as Hedging Instrument [Member] \| Fixed-rate Automotive Loans [Member] \| Interest and Dividends Income [Member] \| Fair Value Hedging [Member] \| Interest Rate Contract [Member]
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
Designated as Hedging Instrument [Member] \| Fixed-rate Automotive Loans [Member] \| Interest Expense [Member] \| Fair Value Hedging [Member] \| Interest Rate Contract [Member]
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

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 25
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986

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Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge 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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a)
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Amount of operating dividend and interest income, including amortization and accretion of premiums and discounts, on available-for-sale securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.2)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 4
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