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REVENUE, CONTRACT ASSETS AND LIABILITIES AND OTHER CONTRACT MATTERS - Narrative (Details) $ in Thousands | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2019 USD ($) | Jun. 30, 2019 USD ($) vessel | Jun. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Long-term Contracts or Programs Disclosure [Line Items] |  |  |  |  |
Bad debt expense |  | $ 59 | $ 8 |  |
Allowance for bad debt | $ 100 | $ 100 |  | $ 400 |
Number of harbor tug vessels | vessel |  | 6 |  |  |
Change in estimated margins | 2,300 | $ 2,000 |  |  |
Number of harbor tug vessels completed | vessel |  | 4 |  |  |
Reduction of estimated contract price for liquidated damages, amount | 11,300 | $ 11,300 |  | $ 11,200 | $ 11,200
Harbor Tug |  |  |  |  |
Long-term Contracts or Programs Disclosure [Line Items] |  |  |  |  |
Change in estimated margins | 1,400 | $ 1,200 |  |  |
Number of harbor tug vessels completed | vessel |  | 6 |  |  |
Reserve for loss | 1,600 | $ 1,600 |  |  |
Ice-Breaker Tug |  |  |  |  |
Long-term Contracts or Programs Disclosure [Line Items] |  |  |  |  |
Change in estimated margins | 900 | 800 |  |  |
Reserve for loss | $ 100 | $ 100 |  |  |

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