Summary of Debt Outstanding (Parenthetical) (Detail) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Troms Offshore Supply AS
Debt [Line Items]
Description of frequency of periodic payments on notes	semi-annually
Weighted average interest rate of debt borrowings	5.00%
Troms Offshore Supply AS \| Level 2
Debt [Line Items]
Fair value of debt outstanding	$ 86.4	$ 88.5
8.00% New Secured Notes Due August 2022
Debt [Line Items]
Debt instrument interest rate	8.00%
Debt Instrument Maturity Period	August 2022
8.00% New Secured Notes Due August 2022 \| Level 2
Debt [Line Items]
Fair value of debt outstanding	$ 361.5	$ 359.8
Norwegian Kroner Denominated Notes Due May 2024 \| Troms Offshore Supply AS
Debt [Line Items]
Debt Instrument Maturity Period	May 2024
Norwegian Kroner Denominated Notes Due January 2026 \| Troms Offshore Supply AS
Debt [Line Items]
Debt Instrument Maturity Period	January 2026
United States Dollar Denominated Notes Due January 2027 \| Troms Offshore Supply AS
Debt [Line Items]
Debt Instrument Maturity Period	January 2027
United States Dollar Denominated Notes Due April 2027 \| Troms Offshore Supply AS
Debt [Line Items]
Debt Instrument Maturity Period	April 2027

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

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