Annual report pursuant to Section 13 and 15(d)

Note 16 - Commitments (Details Textual)

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Note 16 - Commitments (Details Textual) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Escrow Deposit $ 49,500  
Expense Relating to Surety Bonds Paid. 4,700 5,900 $ 5,700
Deposit Liabilities, Collateral Issued, Financial Instruments 6,900    
Surety Bonds [Member]      
Other Commitment, Due in Next Twelve Months 4,600    
Other Commitment, Due in Second Year 4,600    
Other Commitment, Due in Third Year 20,224,600    
Other Commitment, Due in Fourth Year 4,700    
Other Commitment, Due in Fifth Year 4,700    
Other Commitment, Due after Fifth Year 52,000    
Surety Bonds [Member] | Other Commitment [Member]      
Bonding Requirement, Related To Purchase of Properties Minimum Amount 64,000    
Bonding Requirement Related To Purchase of Properties Maximum Amount 94,000    
Surety Bonds [Member] | Total E&P [Member]      
Bonding Requirement Related to Purchase of Properties Amount Current Year. 90,700    
Escrow Deposit 0    
Bonding Requirement Related to Purchase of Properties Amount, Fifth Anniversary. 103,000    
Bonding Requirement Related to Purchase of Properties Amount Increment From Second Anniversary. 3,000    
Surety Bonds [Member] | Exxon [Member]      
Bonding Requirement, Related To Purchase of Properties Minimum Amount 27,300    
Other Commitment, Due in Next Twelve Months 30,000    
Other Commitment, Due in Second Year 33,000    
Other Commitment, Due in Third Year 36,300    
Other Commitment, Due in Fourth Year 40,000    
Other Commitment, Due in Fifth Year 44,000    
Other Commitment, Due after Fifth Year 114,000    
Surety Bonds [Member] | Exxon [Member] | Minimum [Member]      
Annual Increase in Other Commitment 4,000    
Surety Bonds [Member] | Exxon [Member] | Maximum [Member]      
Annual Increase in Other Commitment 9,000    
Surety Bonds [Member] | Conoco [Member]      
Bonding Requirement, Related To Purchase of Properties Minimum Amount 49,000    
Heidelberg Field [Member]      
Other Commitment, Due in Next Twelve Months 3,700    
Other Commitment, Due in Second Year 2,200    
Other Commitment, Due in Third Year 1,600    
Other Commitment, Due in Fourth Year 1,200    
Other Commitment, Due in Fifth Year 800    
Other Commitment, Due after Fifth Year 1,300    
Other Commitment, Expense $ 4,500 $ 2,300