Acquisitions and Divestitures (Details) $ in Thousands, shares in Millions	1 Months Ended		9 Months Ended
	Oct. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) a shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jan. 31, 2018 USD ($)	Dec. 15, 2017 USD ($)	Apr. 17, 2017 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Liabilities related to oil and gas properties sold			$ 28,234	$ 248
Interests In East Lake Verret Field
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from sale of a majority of interests							$ 2,200
Saltwater Disposal Assets In East Texas
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from sale of a majority of interests						$ 8,500
Gulf Of Mexico Properties
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase commitment, remaining minimum amount committed					$ 3,800
Liabilities related to oil and gas properties sold			28,200
Refund receivable			$ 8,300		$ 12,400
Austin Chalk Acreage
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acres acquired | a		24,600
Purchase price		$ 11,100
Common shares issued in connection with acquisition (in shares) | shares		2.0
Subsequent Event | Gulf Of Mexico Properties
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments for previous acquisition	$ 4,200