Note 11 - Commitments and Contingencies (Details Textual) kr in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Feb. 28, 2019 NOK (kr)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 NOK (kr)	Dec. 31, 2017 USD ($)
National Geographic [Member]
Royalty Expense	$ 1.4	$ 1.4
Accounts Payable and Other Accrued Liabilities, Current	1.4			$ 1.0
World Wildlife Fund [Member]
Royalty Expense	0.2	$ 0.2
Class Vessel [Member] | Ulstein Verft [Member]
Vessel, Contract Price					kr 1,066	$ 134.6
Foreign Exchange Contract [Member] | Ulstein Verft [Member]
Vessel, Contract Price	$ 153.5		kr 1,291