Fair Value Measurements and Derivative Instruments - Balance Sheet Hedging Instruments (Details) - Foreign currency debt - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Derivative Instruments
Carrying value of non-derivative instrument designated as hedging instrument	$ 329,809	$ 295,323
Current portion of long-term debt
Derivative Instruments
Carrying value of non-derivative instrument designated as hedging instrument	78,746	70,097
Long-term debt
Derivative Instruments
Carrying value of non-derivative instrument designated as hedging instrument	$ 251,063	$ 225,226