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Comparison of Total Assets (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Corporate Vessels
Segment and Geographic Distribution of Operations [Line Items]
Construction costs	$ 9.5	$ 9.3

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- Definition

Segment and Geographic Distribution of Operations [Line Items]

+ References

No definition available.

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- Definition

The aggregate costs related to construction and development services during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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