Hedging - Notional Amount (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative term	1 month
Designated as Hedging Instrument | Cash Flow Hedging | Foreign currency forward contracts
Derivative [Line Items]
Notional amount of foreign currency forward contract	$ 365,212	$ 253,381
Designated as Hedging Instrument | Cash Flow Hedging | Foreign currency forward contracts | Euro
Derivative [Line Items]
Notional amount of foreign currency forward contract	257,230	164,368
Designated as Hedging Instrument | Cash Flow Hedging | Foreign currency forward contracts | British pound sterling
Derivative [Line Items]
Notional amount of foreign currency forward contract	77,481	65,237
Designated as Hedging Instrument | Cash Flow Hedging | Foreign currency forward contracts | Australian dollar
Derivative [Line Items]
Notional amount of foreign currency forward contract	$ 30,501	$ 23,776
Minimum | Cash Flow Hedging | Foreign currency forward contracts
Derivative [Line Items]
Derivative term	1 month
Maximum | Cash Flow Hedging | Foreign currency forward contracts
Derivative [Line Items]
Derivative term	18 months