Annual report pursuant to Section 13 and 15(d)

Crude Oil and Natural Gas Properties and Equipment (Crude Oil and Natural Gas Properties and Equipment) (Details)

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Crude Oil and Natural Gas Properties and Equipment (Crude Oil and Natural Gas Properties and Equipment) (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Property, Plant and Equipment [Line Items]    
Gross crude oil and natural gas properties and equipment $ 472,800 $ 464,957
Accumulated depreciation, depletion, amortization and impairment (435,764) (396,699)
Net crude oil and natural gas properties, equipment and other 37,036 68,258
Wells, Platforms and Other Production Facilities [Member]    
Property, Plant and Equipment [Line Items]    
Gross crude oil and natural gas properties and equipment 441,879 422,651
Work-in-Progress [Member]    
Property, Plant and Equipment [Line Items]    
Gross crude oil and natural gas properties and equipment 169 7,378
Undeveloped Acreage [Member]    
Property, Plant and Equipment [Line Items]    
Gross crude oil and natural gas properties and equipment 21,476 23,771
Equipment and Other [Member]    
Property, Plant and Equipment [Line Items]    
Gross crude oil and natural gas properties and equipment $ 9,276 $ 11,157