Derivative Financial Instruments and Hedging Activities - Notional Amounts of Outstanding Positions (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Designated as Hedging Instrument | Cash Flow Hedging | Foreign currency exchange contracts
Derivatives, Fair Value [Line Items]
Notional Amount	$ 107,400	$ 44,800
Designated as Hedging Instrument | Cash Flow Hedging | Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Notional Amount	48,000	91,000
Designated as Hedging Instrument | Cash Flow Hedging | Interest rate swaps
Derivatives, Fair Value [Line Items]
Notional Amount	650,000	650,000
Not Designated as Hedging Instrument | Foreign currency exchange contracts
Derivatives, Fair Value [Line Items]
Notional Amount	$ 18,439	$ 0