Derivative Financial Instruments - Fair Value of Derivative Instruments (Details) - Cash Flow Hedge - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Interest rate swap | Other Current Assets [Member]
Derivative Liability
Derivative Asset	$ 616	$ 278
Interest rate swap | Other Noncurrent Assets [Member]
Derivative Liability
Derivative Asset	742	693
Foreign currency hedges | Other Current Liabilities [Member]
Derivative Liability
Derivative Liability	0	(742)
Foreign currency hedges | Other Current Assets [Member]
Derivative Liability
Derivative Asset	864	0
Designated as Hedging Instrument [Member] | Interest rate swap
Derivative Liability
Derivative Asset	1,358	971	$ 753
Designated as Hedging Instrument [Member] | Foreign currency hedges
Derivatives, Fair Value [Line Items]
Foreign Currency Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months	644
Derivative Liability
Derivative Liability		$ (742)	$ (601)
Derivative Asset	$ 864