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Discontinued Operations (Additional Information) (Detail) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) a well	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) a well
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
(Loss) gain on disposition of proved property			$ (13,664)		$ (706)
Net mineral acres leased from others sold \| a			199,263
Gross producing oil and gas wells sold \| well			379
Net producing oil and gas wells sold \| well			95
Proceeds from sale of oil and gas property and equipment			$ 80,374		17,800
Reimbursement of capital costs related to sale of in progress wells			3,269
Non-cash impairment charges			47,172	$ 60,939	$ 108,184
Pending Litigation \| Huffman vs. Forestar Petroleum Corporation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accrual for damages			2,990
Wyoming
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accrual for environmental loss contingencies, revision in estimates		$ 1,043
Environmental liability			1,155
Transaction
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gas and oil area, undeveloped, net \| a					109,000
Productive oil wells, number of wells, gross \| well					39
Productive oil wells, number of wells, net \| well					7
Cost of Sales
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Non-cash impairment charges			$ 0	$ 612	$ 163,029
Discontinued Operations, Disposed of by Sale \| Forestar Petroleum Company
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of stock, consideration received on transaction	$ 100