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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 |  |

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