Summary of Significant Accounting Policies (Details Textual) shares in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($) shares	Jun. 30, 2019 USD ($) shares	Jun. 30, 2020 USD ($) shares	Jun. 30, 2019 USD ($) shares	Jun. 30, 2020 $ / bbl	Jun. 30, 2020 $ / MMBTU
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Capitalized general and administrative costs	$ 8,100,000	$ 8,400,000	$ 16,300,000	$ 16,800,000
Interest costs capitalized	1,800,000	2,600,000	$ 3,200,000	4,200,000
Discount rate, present value of future revenue			10.00%
Average oil and gas prices					43.60	2.07
Full-cost ceiling impairment		0	$ 243,900,000	0
Full-cost ceiling impairment	$ 324,001,000	$ 0	324,001,000	$ 0
Deferred income tax benefit related to impairment charge			$ 80,100,000
Options
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Antidilutive securities (in shares) | shares	2.5	2.8	2.6	2.8
Restricted Stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Antidilutive securities (in shares) | shares	0.5		0.6