Acquisitions Acquistions (Purchase Price Allocation) (Details) - USD ($) $ in Thousands |
Aug. 28, 2018 |
Sep. 30, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|---|---|---|---|---|
| Business Combinations [Abstract] | ||||
| Current assets | $ 166 | |||
| Property, plant & equipment | 17,543 | |||
| Goodwill | 16,777 | $ 304,733 | $ 287,956 | $ 275,487 |
| Intangible and other non-current assets | 3,863 | |||
| Assumed liabilities | 399 | |||
| Purchase price | $ 37,950 |