INVENTORIES - Summary of inventories (Details) - USD ($) $ in Thousands |
Mar. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| INVENTORIES | ||
| Raw materials and supplies | $ 22,922 | $ 20,116 |
| Work in process | 10,450 | 8,300 |
| Finished goods | 10,668 | 9,590 |
| Total inventories | $ 44,040 | $ 38,006 |