Derivative Instruments - Notional Amounts of Derivatives Outstanding (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Notional amount	$ 8,088	$ 6,739
Foreign Exchange Contract | Designated as Hedging Instrument | Cash Flow Hedging
Derivatives, Fair Value [Line Items]
Notional amount	1,936	1,990
Foreign Exchange Contract | Designated as Hedging Instrument | Net Investment Hedging
Derivatives, Fair Value [Line Items]
Notional amount	210	0
Foreign Exchange Contract | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Notional amount	3,542	2,349
Interest Rate Contract | Designated as Hedging Instrument | Fair Value Hedging
Derivatives, Fair Value [Line Items]
Notional amount	$ 2,400	$ 2,400