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Note 6 - Revenues (Details Textual)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2021
USD ($)
Mar. 31, 2020
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Sep. 03, 2014
Number of Suspended Saltwater Injection Wells 2       2
Revenue from Contract with Customer, Excluding Assessed Tax, Total $ 15,113 $ 14,403      
Accounts Receivable, Allowance for Credit Loss, Ending Balance 300   $ 300    
Contract with Customer, Asset, after Allowance for Credit Loss, Total 701 $ 813 585 $ 602  
Contract with Customer, Liability, Total $ 4,100   3,200    
Customer Concentration Risk [Member] | Revenue Benchmark [Member]          
Number of Major Customers   0      
Customer Concentration Risk [Member] | Accounts Receivable [Member]          
Number of Major Customers 1        
Customer Concentration Risk [Member] | Accounts Receivable [Member] | One Customer [Member]          
Concentration Risk, Percentage 20.00%        
Waste Management Services [Member]          
Revenue from Contract with Customer, Excluding Assessed Tax, Total $ 11,150 $ 11,133      
Revenue from Contract with Customer, Excluding Assessed Tax, Percentage 74.00% 77.00%      
Accounts Receivable, after Allowance for Credit Loss, Total $ 8,800   $ 7,900    
Waste Management Services [Member] | Maximum [Member]          
Contract Term (Year) 1 year        
Waste Management Services [Member] | Customer Concentration Risk [Member] | Sales Revenue, Net to External Customers [Member]          
Number of Major Customers 1        
Waste Management Services [Member] | Customer Concentration Risk [Member] | Sales Revenue, Net to External Customers [Member] | One Customer [Member]          
Concentration Risk, Percentage 20.00%        
Waste Management Services [Member] | Customer Concentration Risk [Member] | Revenue Benchmark [Member]          
Number of Major Customers 1 0      
Waste Management Services [Member] | Customer Concentration Risk [Member] | Revenue Benchmark [Member] | One Customer [Member]          
Concentration Risk, Percentage 15.00%        
Waste Management Services [Member] | Customer Concentration Risk [Member] | Accounts Receivable [Member]          
Number of Major Customers 1   0    
Waste Management Services [Member] | Customer Concentration Risk [Member] | Accounts Receivable [Member] | One Customer [Member]          
Concentration Risk, Percentage 25.00%        
Golf and Related Operations [Member]          
Revenue from Contract with Customer, Excluding Assessed Tax, Total $ 3,963 $ 3,270      
Revenue from Contract with Customer, Excluding Assessed Tax, Percentage 26.00% 23.00%      
Number of Major Customers 1,900,000   800,000    
Number of Golf Courses 4        
Golf and Related Operations [Member] | Customer Concentration Risk [Member] | Revenue Benchmark [Member]          
Number of Major Customers 0 0      
Membership Agreement, Term (Year) 1 year        
Golf and Related Operations [Member] | Customer Concentration Risk [Member] | Accounts Receivable [Member]          
Number of Major Customers 0   0    
Salt Water Injection Operations [Member] | Waste Management Services [Member]          
Revenue from Contract with Customer, Excluding Assessed Tax, Total $ 0 $ 0      
Customer Advance Deposits [Member]          
Contract with Customer, Liability, Total $ 752 $ 630 $ 674 $ 553