Derivative Financial Instruments and Hedging Activities - Derivative Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivatives Designated as Hedges [Member] \| Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Notional Amount	$ 5,001	$ 3,070
Derivative Asset, Fair Value	120	12
Derivatives Designated as Hedges [Member] \| Assets [Member] \| Fair Value Hedges [Member] \| Fair Value, Inputs, Level 2 [Member] \| Interest Rate Swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Notional Amount	1,250	0
Derivative Asset, Fair Value	2	0
Derivatives Designated as Hedges [Member] \| Assets [Member] \| Cash Flow Hedges [Member] \| Fair Value, Inputs, Level 2 [Member] \| Foreign Currency Swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Notional Amount	1,574	0
Derivative Asset, Fair Value	103	0
Derivatives Designated as Hedges [Member] \| Assets [Member] \| Cash Flow Hedges [Member] \| Fair Value, Inputs, Level 2 And Level 3 [Member] \| Interest Rate Swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Notional Amount	2,177	3,070
Derivative Asset, Fair Value	15	12
Derivatives Designated as Hedges [Member] \| Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Notional Amount	9,860	8,991
Derivative Liability, Fair Value	290	310
Derivatives Designated as Hedges [Member] \| Liabilities [Member] \| Fair Value Hedges [Member] \| Fair Value, Inputs, Level 2 [Member] \| Interest Rate Swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Notional Amount	9,860	7,700
Derivative Liability, Fair Value	290	276
Derivatives Designated as Hedges [Member] \| Liabilities [Member] \| Cash Flow Hedges [Member] \| Fair Value, Inputs, Level 2 [Member] \| Foreign Currency Swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Notional Amount	0	791
Derivative Liability, Fair Value	0	33
Derivatives Designated as Hedges [Member] \| Liabilities [Member] \| Cash Flow Hedges [Member] \| Fair Value, Inputs, Level 2 And Level 3 [Member] \| Interest Rate Swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Notional Amount	0	500
Derivative Liability, Fair Value	0	1
Derivatives not Designated as Hedges [Member] \| Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Notional Amount	56,782	17,657
Derivative Asset, Fair Value	433	80
Derivatives not Designated as Hedges [Member] \| Assets [Member] \| Fair Value, Inputs, Level 2 [Member] \| Foreign Currency Swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Notional Amount	1,201	0
Derivative Asset, Fair Value	104	0
Derivatives not Designated as Hedges [Member] \| Assets [Member] \| Fair Value, Inputs, Level 2 [Member] \| Interest Rate Caps and Floors [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Notional Amount	16,840	9,698
Derivative Asset, Fair Value	69	26
Derivatives not Designated as Hedges [Member] \| Assets [Member] \| Fair Value, Inputs, Level 2 And Level 3 [Member] \| Interest Rate Swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Notional Amount	38,741	7,959
Derivative Asset, Fair Value	260	54
Derivatives not Designated as Hedges [Member] \| Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Notional Amount	26,357	18,316
Derivative Liability, Fair Value	207	54
Derivatives not Designated as Hedges [Member] \| Liabilities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Interest Rate Caps and Floors [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Notional Amount	17,953	12,146
Derivative Liability, Fair Value	70	26
Derivatives not Designated as Hedges [Member] \| Liabilities [Member] \| Fair Value, Inputs, Level 2 And Level 3 [Member] \| Interest Rate Swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Notional Amount	8,404	6,170
Derivative Liability, Fair Value	$ 137	$ 28

Nominal or face amount used to calculate payments on the derivative asset.

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No definition available.

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