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

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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