Summary of Significant Accounting Policies (Details Textual) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in oil and natural gas revenues	$ (212,426)		$ (397,448)
Restricted cash	21,063	$ 15,040	$ 21,063	$ 15,040	$ 5,977
Discount rate, present value of future revenue			10.00%
Impairment charges		0	$ 0
Capitalized general and administrative costs	6,800	5,200	14,100	10,800
Interest costs capitalized	2,600	1,900	4,500	3,200
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	(2,600)		(4,800)
Decrease in production taxes, transportation and processing expenses	$ 2,600		$ 4,800