Summary of Significant Accounting Policies (Details Textual) $ in Thousands		3 Months Ended		9 Months Ended
	Sep. 12, 2018 USD ($) a	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in oil and natural gas revenues		$ (223,091)		$ (620,539)
Restricted cash		7,066	$ 10,744	$ 7,066	$ 10,744	$ 5,977
Discount rate, present value of future revenue				10.00%
Impairment charges			0	$ 0
Capitalized general and administrative costs		8,500	6,100	22,600	16,900
Interest costs capitalized		1,700	2,100	6,200	5,200
Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restricted cash		6,000	$ 1,300	6,000	$ 1,300
Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in oil and natural gas revenues		(2,800)		(7,600)
Decrease in production taxes, transportation and processing expenses		$ 2,800		$ 7,600
BLM Acquisition
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Leasehold acres | a	8,400
Leasehold acres, net | a	8,400
Consideration transferred	$ 387,000