Hedging - Notional Amount (Details) - Foreign currency forward contracts - Designated as hedging instrument - Cash flow hedging - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional amount of foreign currency forward contract	$ 458,328	$ 365,212
Euro
Derivative [Line Items]
Notional amount of foreign currency forward contract	326,024	257,230
British pound sterling
Derivative [Line Items]
Notional amount of foreign currency forward contract	78,066	77,481
Australian dollar
Derivative [Line Items]
Notional amount of foreign currency forward contract	33,134	30,501
Canadian dollar
Derivative [Line Items]
Notional amount of foreign currency forward contract	$ 21,104	$ 0