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Indebtedness - Senior Debt Notes - Additional Information (Detail) $ in Thousands, kr in Millions |  |  |  |  | 1 Months Ended | 9 Months Ended | 12 Months Ended |  |
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Jul. 31, 2017 USD ($) Vessel | Nov. 15, 2013 USD ($) | Sep. 30, 2013 USD ($) | Aug. 15, 2011 USD ($) | May 27, 2015 USD ($) | Mar. 31, 2015 USD ($) | Jan. 31, 2014 NOK (kr) | May 31, 2012 NOK (kr) | Dec. 31, 2017 NOK (kr) | Mar. 31, 2011 USD ($) | Jun. 30, 2018 USD ($) | Dec. 31, 2017 USD ($)
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount | $ 350,000 |  |  |  |  |  |  |  |  |  |  |
Debt instrument outstanding amount |  |  |  |  |  |  |  |  |  |  | $ 436,403 | $ 438,827
Notes Due May 2024 |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument maturity, month and year |  |  |  |  |  |  |  | 2024-05 |  |  |  |
Aggregate principal amount | kr |  |  |  |  |  |  |  | kr 204.4 |  |  |  |
Indebtedness rate |  |  |  |  |  |  | 1.25% |  |  |  |  |
Indebtedness sub-tranche premium rate |  |  |  |  |  |  | 1.00% |  |  |  |  |
Total capitalization rate |  |  |  |  |  |  | 6.13% |  |  |  |  |
Debt instrument outstanding amount | kr |  |  |  |  |  |  |  |  | kr 115.0 |  |  |
Debt instrument fixed interest rate |  |  |  |  |  |  |  | 3.88% |  |  |  |
September 2013 Senior Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Senior note issuable amount under purchase agreement |  |  | $ 500,000 |  |  |  |  |  |  |  |  |
Sale of debt outstanding |  | $ 200,000 | $ 300,000 |  |  |  |  |  |  |  |  |
September 2013 Senior Unsecured Notes | Minimum |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument maturity, in years |  |  |  |  |  |  |  |  | 7 years |  |  |
September 2013 Senior Unsecured Notes | Maximum |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument maturity, in years |  |  |  |  |  |  |  |  | 12 years |  |  |
August 2011 Senior Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Sale of debt outstanding |  |  |  | $ 165,000 |  |  |  |  |  |  |  |
August 2011 Senior Unsecured Notes | Minimum |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument maturity, in years |  |  |  | 8 years |  |  |  |  |  |  |  |
August 2011 Senior Unsecured Notes | Maximum |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument maturity, in years |  |  |  | 10 years |  |  |  |  |  |  |  |
September 2010 Senior Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Sale of debt outstanding |  |  |  |  |  |  |  |  |  | $ 425,000 |  |
September 2010 Senior Unsecured Notes | Minimum |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument maturity, in years |  |  |  |  |  |  |  |  |  | 5 years |  |
September 2010 Senior Unsecured Notes | Maximum |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument maturity, in years |  |  |  |  |  |  |  |  |  | 12 years |  |
Troms Offshore Supply AS | Notes Due April 2027 |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument maturity, month and year |  |  |  |  | 2027-04 |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | $ 31,300 |  |  |  |  |  |  |
Debt instrument bearing floating interest rate |  |  |  |  | 2.92% |  |  |  |  |  |  |
Indebtedness rate |  |  |  |  | 1.00% |  |  |  |  |  |  |
Indebtedness sub-tranche premium rate |  |  |  |  | 1.00% |  |  |  |  |  |  |
Total capitalization rate |  |  |  |  | 492.00% |  |  |  |  |  |  |
Debt instrument outstanding amount |  |  |  |  |  |  |  |  |  |  |  | 25,500
Troms Offshore Supply AS | Notes Due January 2027 |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument maturity, month and year |  |  |  |  |  | 2027-01 |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  | $ 29,500 |  |  |  |  |  |
Debt instrument bearing floating interest rate |  |  |  |  |  | 2.91% |  |  |  |  |  |
Indebtedness rate |  |  |  |  |  | 1.00% |  |  |  |  |  |
Indebtedness sub-tranche premium rate |  |  |  |  |  | 1.00% |  |  |  |  |  |
Total capitalization rate |  |  |  |  |  | 4.91% |  |  |  |  |  |
Debt instrument outstanding amount |  |  |  |  |  |  |  |  |  |  |  | 23,300
Troms Offshore Supply AS | Notes Due January 2026 |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument maturity, month and year |  |  |  |  |  |  | 2026-01 |  |  |  |  |
Aggregate principal amount | kr |  |  |  |  |  |  | kr 300.0 |  |  |  |  |
Indebtedness rate |  |  |  |  |  |  | 1.25% |  |  |  |  |
Indebtedness sub-tranche premium rate |  |  |  |  |  |  | 1.00% |  |  |  |  |
Total capitalization rate |  |  |  |  |  |  | 4.56% |  |  |  |  |
Debt instrument outstanding amount |  |  |  |  |  |  |  |  | kr 212.5 |  |  | $ 26,000
Troms Offshore Supply AS | Notes Due January 2026 | Minimum |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument bearing floating interest rate |  |  |  |  |  |  | 231.00% |  |  |  |  |
Troms Offshore Supply AS | Plan of Reorganization | Senior Secured Notes |  |  |  |  |  |  |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Plan effective date | Jul. 31, 2017 |  |  |  |  |  |  |  |  |  |  |
Modification description of debt instrument | (i) reduce by 50% the required principal payments due from the Effective Date through March 31, 2019, (ii) modestly increase the interest rates on amounts outstanding through April 2023, and (iii) provide for security and additional guarantees, including (a) mortgages on six vessels and related assignments of earnings and insurances, (b) share pledges by Troms Offshore and certain subsidiaries of Troms Offshore, and (c) guarantees by certain subsidiaries of Troms Offshore. |  |  |  |  |  |  |  |  |  |  |
Percentage of required principal payments due from effective date | 50.00% |  |  |  |  |  |  |  |  |  |  |
Number of vessels mortgaged | Vessel | 6 |  |  |  |  |  |  |  |  |  |  |

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