Note 3 - Vessel Acquisitions, Dispositions and New-Build Program (Details Textual) NOK in Millions	1 Months Ended						3 Months Ended		12 Months Ended
	Apr. 30, 2014 NOK	Apr. 30, 2014 USD ($)	Feb. 28, 2014 USD ($)	Sep. 30, 2013 USD ($)	Aug. 31, 2013 USD ($)	Jan. 31, 2013 USD ($)	Dec. 31, 2015 NOK	Dec. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)
Vessel [Member] | North Sea [Member]
Proceeds from Sale of Property, Plant, and Equipment					$ 10,800,000
Property, Plant and Equipment, Additions			$ 30,900,000
Vessel [Member] | Americas [Member]
Proceeds from Sale of Property, Plant, and Equipment				$ 2,100,000
Vessel [Member]
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property						$ 0				$ 12,500,000	$ 6,000,000
Proceeds from Sale of Property, Plant, and Equipment						$ 700,000				$ 32,300,000
Large Platform Supply Vessel [Member] | North Sea [Member]
Number of Vessels Under Construction													6
Additional PSV [Member] | North Sea [Member]
Number of Ship Yards used in Agreements													1
Platform Supply Vessel[ Member] | North Sea [Member]
Number of Vessels Expected to Operate													7
Estimated Cost Of Building Vessels	NOK 359.0	$ 40,900,000							$ 60,000,000				$ 288,000,000
Installment Payment on Vessel Contracts							NOK 92.2	$ 10,400,000
Final Payment on Vessel Contracts							NOK 195.0	$ 22,100,000
Platform Supply Vessel[ Member] | Americas [Member]
Number of Vessels Expected to Operate												4
Estimated Cost Of Building Vessels												$ 168,000,000
Platform Supply Vessel[ Member] | BAE Systems [Member]
Number of Vessels Under Construction												2
North Sea [Member]
Number of Vessel Sold					2					1
Number of Ship Yards used in Agreements													3
Americas [Member]
Number of Vessels Sold				1
South East Asia [Member]
Number of Vessel Sold										3
Thoma Sea [Member]
Number of Vessels Under Construction												2
Increase (Decrease) in Restricted Cash												$ 52,400,000
Escrow Deposit										$ 3,700,000
Proceeds from Sale of Property, Plant, and Equipment									$ 8,910,000	$ 32,261,000	$ 13,512,000
Number of Vessel Sold										4
Number of Vessels Being Built								12	12
Escrow Deposit										$ 3,683,000