Derivative Instruments and Hedging Activities (Statement of Gains and Losses on Derivative Instruments Reported in Statement of Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	$ 0	$ 0	$ (1)	$ (5)
Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	(3)	0	(7)	(9)
Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments	0	1	(2)	3
Foreign Exchange Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments	(3)	0	(4)	(5)
Equity Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	(1)	0	(2)
Interest Income [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	(1)	(6)	1	(34)
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments	1	0	1	0
Interest Income [Member] | Interest Rate Contract [Member] | Hedges of Retail Auto Loans Held for Investment [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	5	1	12
Interest Income [Member] | Interest Rate Contract [Member] | Losses Related to Amortization of Deferred Basis Adjustments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	6	4	11	9
Interest Income [Member] | Interest Rate Contract [Member] | Gains Related to Amortization of Deferred Basis Adjustments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	20	21	40	39
Interest Income [Member] | Interest Rate Contract [Member] | Losses Related to Amortization of Deferred Basis Adjustments (FHLB) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	1	0	1	0
Interest Income [Member] | Interest Rate Contract [Member] | Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	1	5	(3)	33
Interest Expense [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	20	51	24	242
Interest Expense [Member] | Interest Rate Contract [Member] | Hedges of Unsecured Debt Held for Investment [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	7	11	12	27
Interest Expense [Member] | Interest Rate Contract [Member] | Hedges of Secured Debt [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	2	0	3
Interest Expense [Member] | Interest Rate Contract [Member] | Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	(20)	(50)	(23)	(246)
Interest Expense [Member] | Foreign Exchange Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments	0	(1)	0	(2)
Other Income [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments	(1)	1	(3)	3
Other Income [Member] | Foreign Exchange Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments	(3)	1	(4)	(3)
Other Income [Member] | Foreign Exchange Contract [Member] | Revaluation of Foreign-Denominated Debt or Receivable [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments	4	1	5	3
Employee compensation and benefits [Member] | Equity Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	$ 0	$ (1)	$ 0	$ (2)