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Commitments and Contingencies - Additional Information (Detail) $ in Millions | 1 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Nov. 22, 2017 USD ($) | Dec. 31, 2017 USD ($) T | Mar. 31, 2016 USD ($) | Jun. 30, 2018 USD ($) Vessel T | Jul. 31, 2017 USD ($)
Significant Purchase And Supply Commitment [Line Items] |  |  |  |  |
Cash compensation on termination of employment, maximum |  | $ 68.0 |  |  |
Fast Crew Supply Boat [Member] |  |  |  |  |
Significant Purchase And Supply Commitment [Line Items] |  |  |  |  |
Number of vessels under construction |  | 1 |  |  |
Remaining accumulated cost reclassified from construction in progress to other assets as insurance receivable |  |  | $ 5.6 |  |
Remaining insurance receivable |  | $ 1.8 |  |  | $ 1.8
Vessel Commitments |  |  |  |  |
Significant Purchase And Supply Commitment [Line Items] |  |  |  |  |
Significant commitment, new construction deadweight tons capacity | T |  | 5,400 |  | 5,400 |
'Number of Vessels, commitments |  | 2 |  | 1 |
Settlement agreement, date | November 22, 2017 |  |  |  |
Delivery date of first PSV | 2017-11 |  |  |  |
Final installment paid | $ 4.3 |  |  |  |
Delivery date of second PSV | 2018-07 |  |  |  |
Remaining installment payment for second PSV | $ 4.5 |  |  |  |
Settlement credit for future dry docking services | $ 0.3 |  |  |  |
'Vessel under construction, fair value, after fresh start accounting |  |  |  |  | $ 7.0
Remaining Balance, commitments |  |  |  | $ 2.3 |

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