Sale of Oil and Natural Gas Property Interests - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) a	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) a	Jun. 30, 2017 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of assets	$ 1,553,000	$ (6,000)	$ 1,820,000	$ (1,000)
Asset Sale 1,000 Acres [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of assets	1,500,000		1,500,000
Proceeds from sale of asset	$ 6,000,000.0		$ 6,000,000.0
Area of land | a	1,000		1,000
Asset Sale 400 Acres [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of assets			$ 0
Proceeds from sale of asset			$ 3,800,000
Area of land | a	400		400
Asset Sale 50 Acres [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of assets			$ 300,000
Proceeds from sale of asset			$ 300,000
Area of land | a	50		50