Dispositions - Additional Information (Detail) - USD ($)		3 Months Ended		6 Months Ended		9 Months Ended
	Oct. 15, 2018	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
(Loss) gain on the sale of assets		$ (30,000)	$ 102,000			$ 149,000	$ 23,509,000
Northern Oklahoma
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
(Loss) gain on the sale of assets		(39,000)
Proceeds from sale of oil and gas properties		23,300,000
Miscellaneous Proved, Unproved Properties and Surface Acreage
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
(Loss) gain on the sale of assets		9,000	102,000	$ 179,000	$ 1,300,000
Proceeds from sale of oil and gas properties		$ 673,000	$ 295,000	$ 366,000	$ 1,300,000
Western Oklahoma Property
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
(Loss) gain on the sale of assets							22,100,000
Proceeds from sale of oil and gas properties							$ 26,000,000
Subsequent Event | Pennsylvania
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Percentage of royalty disposed	1.00%
Proceeds from leases	$ 300,000,000