Property, Plant and Equipment - Additional Information (Detail) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Nov. 30, 2011 Vessel Shipyard	Dec. 31, 2017 USD ($) Vessel	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) Vessel $ / shares
Property, Plant and Equipment [Line Items]
Aggregate cost of OSV newbuild program excluding construction period interest | $		$ 1,335,000
Cost incurred on OSV newbuild program | $		$ 1,272,700
Percentage of total project cost		95.30%
NumberofVesselsSold				4
Proceeds from Sale of Other Property, Plant, and Equipment | $				$ 152,000
(Gain) loss on sale of assets | $		$ (121)	$ 54	44,060
Gain (Loss) on Disposition of Assets | $				$ 27,600
Gain (Loss) on Disposition of Assets, net of tax per share | $ / shares				$ 0.76
Newbuild program 5
Property, Plant and Equipment [Line Items]
Number Of Construction Shipyards, Domestic | Shipyard	3
Number of Vessels Placed in Service		22
Number Of Vessels	24	2
Aggregate cost of OSV newbuild program excluding construction period interest | $		$ 1,335,000
Cost incurred on OSV newbuild program | $		$ 1,272,700
Percentage of total project cost		95.30%
Newbuild program 5 | Offshore Supply Vessel Class 300
Property, Plant and Equipment [Line Items]
Number of vessels to be constructed	4
Newbuild program 5 | Offshore Supply Vessel Class 310
Property, Plant and Equipment [Line Items]
Number of vessels to be constructed	5
Newbuild program 5 | Offshore Supply Vessel Class 320
Property, Plant and Equipment [Line Items]
Number of vessels to be constructed	10
Newbuild program 5 | Multi Purpose Supply Vessel Class 310
Property, Plant and Equipment [Line Items]
Number of vessels to be constructed	3
Newbuild program 5 | Multi Purpose Supply Vessel Class 400
Property, Plant and Equipment [Line Items]
Number of vessels to be constructed	2