Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 4,427,810	$ 4,300,666
Accumulated depletion and depreciation and impairment	(3,433,173)	(3,336,184)
Total Property, Plant and Equipment (Note 4)	994,637	964,482
Right-of-use asset, gross	11,700	11,400
Right-of-use asset, net	2,500	4,400
Oil and natural gas properties
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,395,031	4,268,531
Proved
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,236,548	4,106,768
Unproved
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	158,483	161,763
Other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 32,779	$ 32,135