Derivative Financial Instruments and Hedging Activities (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Effect of Derivative Financial Instruments [Abstract]
Derivative, Gain (Loss) on Derivative, Net	$ 227	$ 939	$ 481
Balance Sheet Effect of Derivative Financial Instruments [Abstract]
Notional	94,924	99,856
Fair Value of Derivative Assets	935	909
Fair Value of Derivative Liabilities	310	166
Derivative, Collateral, Obligation to Return Cash	15	15
Derivative, Collateral, Right to Reclaim Cash	38	12
Derivative Asset, Not Offset, Policy Election Deduction	162	113
Interest Rate Contract [Member] \| Not Designated as Hedging Instrument [Member]
Income Effect of Derivative Financial Instruments [Abstract]
Derivative, Gain (Loss) on Derivative, Net	58	(9)	(58)
Balance Sheet Effect of Derivative Financial Instruments [Abstract]
Notional	60,504	61,689
Interest Rate Contract [Member] \| Fair Value Hedging [Member] \| Designated as Hedging Instrument [Member]
Income Effect of Derivative Financial Instruments [Abstract]
Net interest settlements and accruals excluded from the assessment of hedge effectiveness	217	367	370
Ineffectiveness	(1)	4	3
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	(268)	(120)	72
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	267	124	(69)
Balance Sheet Effect of Derivative Financial Instruments [Abstract]
Notional	28,008	33,175
Foreign Exchange Contract [Member] \| Not Designated as Hedging Instrument [Member]
Income Effect of Derivative Financial Instruments [Abstract]
Derivative, Gain (Loss) on Derivative, Net	(150)	179	66
Balance Sheet Effect of Derivative Financial Instruments [Abstract]
Notional	2,406	1,791
Cross Currency Interest Rate Contract [Member] \| Not Designated as Hedging Instrument [Member]
Income Effect of Derivative Financial Instruments [Abstract]
Derivative, Gain (Loss) on Derivative, Net	103	398	$ 100
Balance Sheet Effect of Derivative Financial Instruments [Abstract]
Notional	4,006	3,201
Level 2 [Member] \| Fair Value, Measurements, Recurring [Member] \| Interest Rate Contract [Member] \| Not Designated as Hedging Instrument [Member]
Balance Sheet Effect of Derivative Financial Instruments [Abstract]
Fair Value of Derivative Assets	276	156
Fair Value of Derivative Liabilities	137	74
Level 2 [Member] \| Fair Value, Measurements, Recurring [Member] \| Interest Rate Contract [Member] \| Fair Value Hedging [Member] \| Designated as Hedging Instrument [Member]
Balance Sheet Effect of Derivative Financial Instruments [Abstract]
Fair Value of Derivative Assets	248	487
Fair Value of Derivative Liabilities	135	80
Level 2 [Member] \| Fair Value, Measurements, Recurring [Member] \| Foreign Exchange Contract [Member] \| Not Designated as Hedging Instrument [Member]
Balance Sheet Effect of Derivative Financial Instruments [Abstract]
Fair Value of Derivative Assets	3	24
Fair Value of Derivative Liabilities	10	4
Level 2 [Member] \| Fair Value, Measurements, Recurring [Member] \| Cross Currency Interest Rate Contract [Member] \| Not Designated as Hedging Instrument [Member]
Balance Sheet Effect of Derivative Financial Instruments [Abstract]
Fair Value of Derivative Assets	408	242
Fair Value of Derivative Liabilities	$ 28	$ 8

Balance Sheet Effect of Derivative Financial Instruments [Abstract]

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Income Effect of Derivative Financial Instruments [Abstract]

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Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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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 the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

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Fair value of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be offset, deducted from derivative assets.

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Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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The net gain (loss) included in earnings for the period relating to components of the gain (loss) on interest rate fair value hedging instruments excluded from the assessment of fair value hedge effectiveness.

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The net gain (loss) recognized in earnings during the period due to the ineffectiveness on interest rate fair value hedges.

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