Derivative Instruments and Hedging Activities (Interest and Amortization on Derivative Instruments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Interest and Fee Income, Loans and Leases	$ 1,543	$ 1,368
Interest and Dividend Income, Securities, Operating, Available-for-sale	176	134
Interest Expense, Long-term Debt	411	424
Earnings on cash flow hedges to be recognized within twelve months	13
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	(2)
Designated as Hedging Instrument [Member] | Interest and Fees on Finance Receivables and Loans [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	(11)	(6)
Designated as Hedging Instrument [Member] | Interest and Fees on Finance Receivables and Loans [Member] | Amortization and Interest [Member] | Cash Flow Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Interest for Qualifying Hedge	0	0
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	0	0
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
Designated as Hedging Instrument [Member] | Interest and Dividends Income [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	(1)	0
Designated as Hedging Instrument [Member] | Interest and Dividends Income [Member] | Amortization and Interest [Member] | Cash Flow Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Interest for Qualifying Hedge	0	0
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	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	1	1
Designated as Hedging Instrument [Member] | Interest Expense [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	19	24
Designated as Hedging Instrument [Member] | Interest Expense [Member] | Amortization and Interest [Member] | Cash Flow Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Interest for Qualifying Hedge	1	0
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	1	0
Designated as Hedging Instrument [Member] | Unsecured Debt [Member] | Interest and Fees on Finance Receivables and Loans [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Amortization of Deferred Basis Adjustments	0	0
Gain (Loss) on Interest for Qualifying Hedge	0	0
Designated as Hedging Instrument [Member] | Unsecured Debt [Member] | Interest and Dividends Income [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Amortization of Deferred Basis Adjustments	0	0
Gain (Loss) on Interest for Qualifying Hedge	0	0
Designated as Hedging Instrument [Member] | Unsecured Debt [Member] | Interest Expense [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Amortization of Deferred Basis Adjustments	15	20
Gain (Loss) on Interest for Qualifying Hedge	3	5
Designated as Hedging Instrument [Member] | Federal Home Loan Bank Certificates and Obligations (FHLB) [Member] | Interest and Fees on Finance Receivables and Loans [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Amortization of Deferred Basis Adjustments	0	0
Gain (Loss) on Interest for Qualifying Hedge	0	0
Designated as Hedging Instrument [Member] | Federal Home Loan Bank Certificates and Obligations (FHLB) [Member] | Interest and Dividends Income [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Amortization of Deferred Basis Adjustments	0	0
Gain (Loss) on Interest for Qualifying Hedge	0	0
Designated as Hedging Instrument [Member] | Federal Home Loan Bank Certificates and Obligations (FHLB) [Member] | Interest Expense [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Amortization of Deferred Basis Adjustments	(1)	(1)
Gain (Loss) on Interest for Qualifying Hedge	2	0
Designated as Hedging Instrument [Member] | Available-for-sale Securities [Member] | Interest and Fees on Finance Receivables and Loans [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Interest for Qualifying Hedge	0	0
Designated as Hedging Instrument [Member] | Available-for-sale Securities [Member] | Interest and Dividends Income [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Interest for Qualifying Hedge	(1)	0
Designated as Hedging Instrument [Member] | Available-for-sale Securities [Member] | Interest Expense [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Interest for Qualifying Hedge	0	0
Designated as Hedging Instrument [Member] | Fixed-rate Automotive Loans [Member] | Interest and Fees on Finance Receivables and Loans [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Amortization of Deferred Basis Adjustments	(4)	(5)
Gain (Loss) on Interest for Qualifying Hedge	(7)	(1)
Designated as Hedging Instrument [Member] | Fixed-rate Automotive Loans [Member] | Interest and Dividends Income [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Amortization of Deferred Basis Adjustments	0	0
Gain (Loss) on Interest for Qualifying Hedge	0	0
Designated as Hedging Instrument [Member] | Fixed-rate Automotive Loans [Member] | Interest Expense [Member] | Amortization and Interest [Member] | Fair Value Hedging [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Amortization of Deferred Basis Adjustments	0	0
Gain (Loss) on Interest for Qualifying Hedge	$ 0	$ 0