Summary of Significant Accounting Policies - Estimated Useful Lives by Classification (Detail)	12 Months Ended
	Dec. 31, 2017
Maximum
Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Salvage Value, Percentage	25.00%
Offshore Supply Vessels
Significant Accounting Policies [Line Items]
Estimated useful life	25 years
Multi Purpose Support Vessel
Significant Accounting Policies [Line Items]
Estimated useful life	25 years
Non-vessel related property, plant and equipment | Minimum
Significant Accounting Policies [Line Items]
Estimated useful life	3 years
Non-vessel related property, plant and equipment | Maximum
Significant Accounting Policies [Line Items]
Estimated useful life	28 years