Accrued Liabilities - Components of Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Accrued Liabilities
Accrued product related costs	$ 6,280	$ 5,297
Accrued income taxes	3,937	5,475
Accrued property and other taxes	2,062	997
Accrued professional fees	3,441	2,228
Contract with Customer, Liability	2,810	1,595
Dividends payable	1,323	1,318
Remediation reserves	13,831	17,067	$ 18,176
Other accrued liabilities	6,616	7,367
Total accrued liabilities	$ 40,300	$ 41,344