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Note 9 - Commitments and Contingencies (Details Textual)
kr in Millions
12 Months Ended
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2020
USD ($)
Feb. 25, 2019
NOK (kr)
Nov. 30, 2017
NOK (kr)
Nov. 30, 2017
USD ($)
Increase in Carrying Amount of Redeemable Preferred Stock $ 7,215,000            
Operating Lease, Weighted Average Remaining Lease Term 5 years 120 days            
Operating Lease, Expense $ 1,700,000 $ 1,500,000 $ 1,200,000        
Selling and Marketing Expense, Total 54,772,000 46,987,000 42,354,000        
Letters of Credit Outstanding, Amount 1,200,000 1,200,000          
Perpetual Royalty Agreement [Member]              
Royalty Expense 800,000 700,000 700,000        
Ulstein Verft [Member] | Class Vessel [Member]              
Vessel, Contract Price           kr 1,066 $ 134,600,000
Ulstein Verft [Member] | Foreign Exchange Contract [Member]              
Vessel, Contract Price | kr         kr 1,291    
National Geographic [Member]              
Royalty Expense 5,800,000 5,000,000 4,500,000        
Accounts Payable and Other Accrued Liabilities, Current 2,200,000 1,000,000          
World Wildlife Fund [Member]              
Selling and Marketing Expense, Total 900,000 $ 800,000 $ 600,000        
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            
Natural Habitat, Inc [Member] | Forecast [Member] | Mr. Bressler [Member]              
Share-based Compensation Arrangement by Share-based Payment Award, Options, Final Year Equity Value Threshold       $ 25,000,000      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Fair Value, Percentage of Excess Financial Performance       10.10%      
Mr. Bressler [Member] | Natural Habitat, Inc [Member]              
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 19.90%