Note 10 - Commitments and Contingencies (Details Textual) $ / shares in Units, kr in Millions		12 Months Ended
	Jul. 08, 2015 USD ($) $ / shares shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Nov. 30, 2017 NOK (kr)	Nov. 30, 2017 USD ($)
Operating Leases, Rent Expense, Net, Total		$ 1,300,000	$ 1,200,000	$ 1,100,000
Selling and Marketing Expense, Total		46,987,000	42,354,000	39,072,000
Letters of Credit Outstanding, Amount		1,200,000	1,200,000
License Agreement with National Geographic [Member]
License Agreement Asset		3,600,000	6,500,000
Perpetual Royalty Agreement [Member]
Royalty Expense		700,000	700,000	700,000
National Geographic [Member]
Royalty Expense		5,000,000	4,500,000	4,200,000
Accounts Payable and Other Accrued Liabilities, Current		1,000,000	1,000,000
Number of Shares Called by Options | shares	2,387,499
Call Options, Long Term Asset	$ 13,800,000
Investment Options, Exercise Price | $ / shares	$ 5.76
Selling and Marketing Expense, Total		2,900,000	2,900,000	2,900,000
World Wildlife Fund [Member]
Selling and Marketing Expense, Total		800,000	$ 600,000	$ 500,000
United States Tour Operators Association [Member]
Letters of Credit Outstanding, Amount		1,000,000
Unrelated Insurance Company [Member]
Letters of Credit Outstanding, Amount		$ 150,000
Class Vessel [Member] | Ulstein Verft [Member]
Vessel, Contract Price					kr 1,066	$ 134,600,000