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Note 1 - Nature of Operations and Summary of Significant Accounting Policies (Details Textual)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2021
USD ($)
shares
Dec. 31, 2021
shares
Dec. 31, 2020
shares
Dec. 31, 2019
USD ($)
shares
Jan. 01, 2020
USD ($)
Highly Liquid Investments Maturity Period (Year)   3 years      
Inspection Interval (Year)   5 years      
Increase Of Amortization Period (Month)   30 months      
Number Of Offshore Supply Vessels   153 172    
Number Of Vessels Disposed   18 23    
Number of Ocean Going Tugs, Contracted To Build 2 2      
Ocean Going Tugs, Expected Cost, Each $ 6.3        
Down Payments To Start Construction On Tugboats $ 2.3        
Accounts Receivable, Allowance for Credit Loss, Ending Balance       $ 2.7  
Accounts Receivable, Allowance for Credit Loss, Writeoff       $ 2.6  
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 5,923,399  
Marine Equipment [Member]          
Property, Plant and Equipment, Salvage Value, Percentage 7.50% 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   135      
Actively Employed Vessels [Member]          
Number of Vessels   126      
Stacked Vessels [Member]          
Number of Vessels   9 35