Summary of Property and Equipment (Detail)	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($) $ / bbl	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) $ / bbl	Dec. 31, 2019 USD ($) $ / bbl	Dec. 31, 2018 USD ($) $ / bbl	Dec. 31, 2017 USD ($)
Property, Plant and Equipment [Abstract]
Proved Oil and Gas Property, Full Cost Method	$ 1,545,910,000		$ 1,545,910,000	$ 1,409,219,000
Undeveloped Leasehold Costs Transferred			8,300,000	16,800,000
Interest Costs Capitalized			$ 2,700,000	$ 4,100,000	$ 9,100,000
Amortization Expense Per Physical Unit of Production | $ / bbl	15.83		15.83	17.25	16.11
Unproved Oil and Gas Property excluded	$ 49,900,000		$ 49,900,000	$ 53,200,000
Wells-in-progress	1,200,000		1,200,000
Asset Impairment Charges	120,300,000	$ 271,500,000	391,849,000	0	$ 0
Capitalized Costs of Unproved Properties Excluded from Amortization, Cumulative	2,200,000		2,200,000	1,600,000	2,000,000.0	$ 44,200,000
Capitalized Interest Remaining	1,100,000		1,100,000
Capitalized Costs, Proved Properties	2,100,000		2,100,000	4,100,000	$ 3,700,000
Oil and gas properties:
Unproved Oil and Gas Property, Full Cost Method	49,935,000		49,935,000	53,200,000
Oil and Gas Property, Full Cost Method, Gross	1,595,845,000		1,595,845,000	1,462,419,000
Other property and equipment	27,746,000		27,746,000	25,915,000
Total property and equipment	1,623,591,000		1,623,591,000	1,488,334,000
Accumulated depreciation, depletion and amortization	(900,042,000)		(900,042,000)	(367,909,000)
Property and equipment, net (successful efforts method)	$ 723,549,000		$ 723,549,000	$ 1,120,425,000