Hedging - Notional Amount (Details) - Foreign currency forward contracts - Designated as hedging instrument - Cash flow hedging - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional amount of foreign currency forward contract	$ 453,359	$ 365,212
Euro
Derivative [Line Items]
Notional amount of foreign currency forward contract	317,429	257,230
British pound sterling
Derivative [Line Items]
Notional amount of foreign currency forward contract	71,288	77,481
Australian dollar
Derivative [Line Items]
Notional amount of foreign currency forward contract	31,853	30,501
Canadian dollar
Derivative [Line Items]
Notional amount of foreign currency forward contract	$ 32,789	$ 0