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Revenue, Contract Assets and Liabilities and Other Contract Matters - Additional Information (Details) $ in Millions | 3 Months Ended | 6 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2020 USD ($) | Jun. 30, 2019 USD ($) Vechicle | Jun. 30, 2020 USD ($) Vechicle | Jun. 30, 2019 USD ($) Vechicle | Mar. 31, 2021 | Dec. 31, 2020 | Sep. 30, 2020 | Dec. 31, 2019 USD ($) | Dec. 31, 2017 USD ($)
Long Term Contracts Or Programs Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Reduction of estimated contract price for liquidated damages, amount | $ 11.6 |  | $ 11.6 |  |  |  |  | $ 12.9 | $ 11.2
Retainage |  |  |  |  |  |  |  | $ 3.0 |
Harbor Tug |  |  |  |  |  |  |  |  |
Long Term Contracts Or Programs Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Change in estimated margins |  | $ 1.4 | $ 0.6 | $ 1.2 |  |  |  |  |
Number of vehicle ferry projects | Vechicle |  |  | 2 |  |  |  |  |  |
Reserve for loss | 0.6 | $ 1.6 | $ 0.6 | $ 1.6 |  |  |  |  |
Forty-Vehicle Ferry |  |  |  |  |  |  |  |  |
Long Term Contracts Or Programs Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Change in estimated margins |  |  | $ 1.2 |  |  |  |  |  |
Number of vehicle ferry projects | Vechicle |  | 2 | 2 | 2 |  |  |  |  |
Reserve for loss | 2.5 |  | $ 2.5 |  |  |  |  |  |
Jacket and Deck |  |  |  |  |  |  |  |  |
Long Term Contracts Or Programs Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Change in estimated margins | 0.5 |  | 0.5 |  |  |  |  |  |
Reserve for loss | 0.1 |  | 0.1 |  |  |  |  |  |
Paddlewheel Riverboat and Subsea Components Projects |  |  |  |  |  |  |  |  |
Long Term Contracts Or Programs Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Change in estimated margins | 0.5 |  | 1.4 |  |  |  |  |  |
Ice-Breaker Tug |  |  |  |  |  |  |  |  |
Long Term Contracts Or Programs Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Change in estimated margins |  | $ 0.9 |  | $ 0.8 |  |  |  |  |
Reserve for loss |  | 0.1 |  | 0.1 |  |  |  |  |
Forecast | Harbor Tug |  |  |  |  |  |  |  |  |
Long Term Contracts Or Programs Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Projects, percent complete (percentage) |  |  |  |  |  | 80.00% | 93.00% |  |
Forecast | Forty-Vehicle Ferry |  |  |  |  |  |  |  |  |
Long Term Contracts Or Programs Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Projects, percent complete (percentage) |  |  |  |  | 60.00% | 62.00% |  |  |
Forecast | Jacket and Deck |  |  |  |  |  |  |  |  |
Long Term Contracts Or Programs Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Projects, percent complete (percentage) |  |  |  |  |  |  | 94.00% |  |
Shipyard |  |  |  |  |  |  |  |  |
Long Term Contracts Or Programs Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Change in estimated margins | 0.6 | $ 2.3 | 1.8 | $ 2.0 |  |  |  |  |
Fabrication & Services |  |  |  |  |  |  |  |  |
Long Term Contracts Or Programs Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Change in estimated margins | $ 1.0 |  | $ 1.9 |  |  |  |  |  |

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