Note 4 - Vessel Acquisitions and Dispositions (Details Textual) $ in Thousands, NOK in Millions	3 Months Ended					6 Months Ended
	Jun. 30, 2017 USD ($)	Mar. 31, 2017 NOK	Mar. 31, 2017 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)
Interest Costs Capitalized	$ 0			$ 700		$ 100	$ 1,700
Number of Vessels Under Construction					2	0
Platform Supply Vessel[ Member] | North Sea [Member]
Payments for Installment on Vessel Contracts		NOK 195	$ 23,300
BAE Systems [Member]
Price of Vessels, Option or Right, Not Obligation to Purchase, Exercised					$ 26,000