Note 1 - Business (Details Textual) $ in Thousands	1 Months Ended				9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($)	Apr. 30, 2021 USD ($)	Apr. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2021 USD ($)	Jan. 31, 2022	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Number of Expedition Ships Operated					10	10
Number of Seasonal Charter Vessels Operated					5	5
Cash and Cash Equivalents, at Carrying Value, Ending Balance					$ 150,753	$ 150,753			$ 187,531	$ 101,579
Restricted Cash and Investments, Current, Total					21,940	21,940			16,984	$ 7,679
Proceeds from Coronavirus Economic Relief for Transportation Services (CERTS) Act, Grant						27,000
Long-term Debt, Gross					558,522	$ 558,522			$ 496,492
Second Export Credit Agreement [Member]
Long-term Debt, Gross	$ 122,800
Proceeds from Issuance of Debt	$ 46,200	$ 15,500	$ 30,600	$ 30,500	$ 61,700
First Export Credit Facility [Member]
Long-Term Debt, Aggregate Amount Deferred for Future Payments								$ 15,700
Subsequent Event [Member]
Number of Expedition Ships Operated							9