Schedule of Aggregate Amount of Senior Unsecured Notes Outstanding (Detail) - USD ($) $ in Thousands		12 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Troms Offshore Supply AS
Debt [Line Items]
Amount outstanding		$ 92,000
September 2013 Senior Unsecured Notes
Debt [Line Items]
Aggregate debt outstanding		$ 500,000	$ 500,000
Weighted average remaining life in years		6 years 4 months 24 days	7 years 4 months 24 days
Weighted average coupon rate on notes outstanding		4.86%	4.86%
Fair value of debt outstanding		$ 280,000	$ 342,746
August 2011 Senior Unsecured Notes
Debt [Line Items]
Aggregate debt outstanding		$ 165,000	$ 165,000
Weighted average remaining life in years		3 years 7 months 6 days	4 years 7 months 6 days
Weighted average coupon rate on notes outstanding		4.42%	4.42%
Fair value of debt outstanding		$ 92,400	$ 127,148
September 2010 Senior Unsecured Notes
Debt [Line Items]
Aggregate debt outstanding		$ 382,500	$ 382,500
Weighted average remaining life in years		3 years 1 month 6 days	4 years 1 month 6 days
Weighted average coupon rate on notes outstanding		4.35%	4.35%
Fair value of debt outstanding		$ 214,200	$ 302,832
May 2015 notes | Troms Offshore Supply AS
Debt [Line Items]
Fair value of debt outstanding	[1]	27,395	30,062
Amount outstanding	[1]	27,421	30,033
March 2015 notes | Troms Offshore Supply AS
Debt [Line Items]
Fair value of debt outstanding	[1]	24,544	27,027
Amount outstanding	[1]	$ 24,573	$ 27,030

[1] Note requires semi-annual principal payments.