Crude Oil and Natural Gas Properties and Equipment (Crude Oil and Natural Gas Properties and Equipment) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Gross crude oil and natural gas properties and equipment	$ 525,579	$ 472,800
Accumulated depreciation, depletion, amortization and impairment	(451,477)	(435,764)
Net crude oil and natural gas properties, equipment and other	74,102	37,036
Wells, Platforms and Other Production Facilities [Member]
Property, Plant and Equipment [Line Items]
Gross crude oil and natural gas properties and equipment	480,872	441,879
Work-in-Progress [Member]
Property, Plant and Equipment [Line Items]
Gross crude oil and natural gas properties and equipment	2,278	169
Undeveloped Acreage [Member]
Property, Plant and Equipment [Line Items]
Gross crude oil and natural gas properties and equipment	23,735	21,476
Equipment and Other [Member]
Property, Plant and Equipment [Line Items]
Gross crude oil and natural gas properties and equipment	$ 18,694	$ 9,276

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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