Fair Value Measurements and Derivative Instruments - Balance Sheet Hedging Instruments (Details) - Foreign currency debt - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Derivative Instruments
Carrying value of non-derivative instrument designated as hedging instrument	$ 298,854	$ 295,323
Current portion of long-term debt
Derivative Instruments
Carrying value of non-derivative instrument designated as hedging instrument	79,575	70,097
Long-term debt
Derivative Instruments
Carrying value of non-derivative instrument designated as hedging instrument	$ 219,279	$ 225,226