Investments in affiliates and investments in available-for-sale securities - Investments in available-for-sale securities (Details) - USD ($) $ in Thousands |
3 Months Ended | 9 Months Ended | 12 Months Ended | ||
|---|---|---|---|---|---|
Sep. 30, 2018 |
Sep. 30, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Schedule Of Equity Method Investments [Line Items] | |||||
| Reclassification to earnings | $ 0 | $ 345 | $ 1,782 | ||
| Investment in available-for-sale-securities | $ 227 | $ 227 | 238 | 0 | |
| Unrealized gain | 2 | 0 | |||
| Unrealized loss | $ (11) | $ (2) | |||
| Unrealized gain | $ 16 | ||||
| KLC and STX | |||||
| Schedule Of Equity Method Investments [Line Items] | |||||
| Consideration received for AFS securities sold | 5,303 | ||||
| Korea Line Corporation (''KLC'') | |||||
| Schedule Of Equity Method Investments [Line Items] | |||||
| Reclassification to earnings | $ 345 | $ 1,783 | |||