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Schedule of Aggregate Amount of Senior Unsecured Notes Outstanding (Detail) - USD ($) $ in Thousands | 12 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2017 | Jun. 30, 2018 | Dec. 31, 2017
Debt [Line Items] |  |  |
Amount outstanding |  | $ 436,403 | $ 438,827
Predecessor |  |  |
Debt [Line Items] |  |  |
Aggregate debt outstanding | $ 500,000 |  |
Weighted average remaining life in years | 6 years 4 months 24 days |  |
Weighted average coupon rate on notes outstanding | 4.86% |  |
Fair value of debt outstanding | $ 280,000 |  |
September 2010 Senior Unsecured Notes | Predecessor |  |  |
Debt [Line Items] |  |  |
Aggregate debt outstanding | $ 382,500 |  |
Weighted average remaining life in years | 3 years 1 month 6 days |  |
Weighted average coupon rate on notes outstanding | 4.35% |  |
Fair value of debt outstanding | $ 214,200 |  |
August 2011 Senior Unsecured Notes | Predecessor |  |  |
Debt [Line Items] |  |  |
Aggregate debt outstanding | $ 165,000 |  |
Weighted average remaining life in years | 3 years 7 months 6 days |  |
Weighted average coupon rate on notes outstanding | 4.42% |  |
Fair value of debt outstanding | $ 92,400 |  |
Troms Offshore Supply AS |  |  |
Debt [Line Items] |  |  |
Weighted average coupon rate on notes outstanding |  | 5.00% |
Troms Offshore Supply AS | May 2015 notes |  |  |
Debt [Line Items] |  |  |
Amount outstanding |  |  | 25,463
Fair value of debt outstanding |  |  | 25,427
Troms Offshore Supply AS | May 2015 notes | Predecessor |  |  |
Debt [Line Items] |  |  |
Amount outstanding | 27,421 |  |
Fair value of debt outstanding | 27,395 |  |
Troms Offshore Supply AS | March 2015 notes |  |  |
Debt [Line Items] |  |  |
Amount outstanding |  |  | 23,345
Fair value of debt outstanding |  |  | $ 23,251
Troms Offshore Supply AS | March 2015 notes | Predecessor |  |  |
Debt [Line Items] |  |  |
Amount outstanding | 24,573 |  |
Fair value of debt outstanding | $ 24,544 |  |

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