Note 1 - Nature of Operations and Summary of Significant Accounting Policies (Details Textual) $ in Millions	12 Months Ended
	Dec. 31, 2020 shares	Dec. 31, 2019 shares	Jan. 01, 2020 USD ($)	Dec. 31, 2018 shares
Highly Liquid Investments Maturity Period (Year)	3 years
Inspection Interval (Year)	5
Increase Of Amortization Period (Month)	30 months
Number Of Offshore Supply Vessels	172	217
Number Of Vessels Disposed	23	46
Number Of Assets Held For Sale	23
Accounting Standards Update 2016-13 [Member]
Accounts Receivable, Allowance for Credit Loss, Ending Balance \| $			$ 0.2
Out-of-the-money Warrants [Member]
Class of Warrant or Right, Outstanding (in shares) \| shares	5,923,399	5,923,399		5,923,399
Marine Equipment [Member]
Property, Plant and Equipment, Salvage Value, Percentage	7.50%
Marine Equipment [Member] \| Minimum [Member]
Property, Plant and Equipment, Useful Life (Year)	10 years
Marine Equipment [Member] \| Maximum [Member]
Property, Plant and Equipment, Useful Life (Year)	20 years
Property, Plant and Equipment, Other Types [Member] \| Minimum [Member]
Property, Plant and Equipment, Useful Life (Year)	3 years
Property, Plant and Equipment, Other Types [Member] \| Maximum [Member]
Property, Plant and Equipment, Useful Life (Year)	10 years
Remaining Number Of Vessels [Member]
Number of Vessels	149
Actively Employed Vessels [Member]
Number of Vessels	114
Stacked Vessels [Member]
Number of Vessels	35	19