Cash and Cash Equivalents (Table) (Details) - USD ($) $ in Thousands |
Sep. 30, 2018 |
Dec. 31, 2017 |
Sep. 30, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
|---|---|---|---|---|---|---|
| CASH AND CASH EQUIVALENTS [Abstract] | ||||||
| Cash on hand and at banks | $ 127,625 | $ 126,584 | ||||
| Short-term deposits and highly liquid funds | 7 | 9,408 | ||||
| Cash and cash equivalents | $ 132,925 | $ 127,632 | $ 114,198 | $ 135,992 | $ 163,412 | $ 247,556 |