Hedging - Notional Amount (Details) - Foreign currency forward contracts - Designated as hedging instruments - Cash flow hedging - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional amount of foreign currency forward contract	$ 467,624	$ 365,212
Euro
Derivative [Line Items]
Notional amount of foreign currency forward contract	324,753	257,230
British pound sterling
Derivative [Line Items]
Notional amount of foreign currency forward contract	74,494	77,481
Canadian dollar
Derivative [Line Items]
Notional amount of foreign currency forward contract	37,098	0
Australian dollar
Derivative [Line Items]
Notional amount of foreign currency forward contract	$ 31,279	$ 30,501