Schedule of Vessel Commitments (Detail) $ in Thousands		1 Months Ended	6 Months Ended
		May 31, 2015 USD ($) Vessel	Sep. 30, 2016 USD ($) Vessel
Vessel Commitments
Significant Purchase and Supply Commitment [Line Items]
Number of Vessels, commitments | Vessel	[1]		3
Total cost, commitments	[1]		$ 163,657
Invested, commitments	[1]		115,683
Remaining Balance, commitments	[1]		$ 47,974
Deepwater PSVs
Significant Purchase and Supply Commitment [Line Items]
Number of vessels under construction | Vessel		2	3	[2]
Total cost, under construction	[2]		$ 163,657
Invested, under construction		$ 5,400	115,683	[2]
Remaining Balance, under construction	[2]		$ 47,974

[1] The company is entitled to receive a refund of prior shipyard payments totaling $14.3 million which would reduce the remaining balance of vessel commitments. See further discussion below.

[2] Six additional option vessels and a fast supply boat are not included in the table above.