Investments in affiliates and investments in available-for-sale securities - Investments in available-for-sale securities (Details) - USD ($) $ in Thousands |
12 Months Ended | ||
|---|---|---|---|
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 | 238 | 0 | |
| Unrealized gain | $ 2 | 0 | |
| 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 | |