Summary of Significant Accounting Policies (Details Textual) shares in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) shares	Sep. 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	$ 7,600,000	$ 8,300,000		$ 23,900,000	$ 25,100,000
Interest costs capitalized	2,000,000.0	2,700,000		$ 5,200,000	7,000,000.0
Discount rate, present value of future revenue				10.00%
Average oil and gas prices						39.71	1.97
Full-cost ceiling impairment	189,100,000	0	$ 243,900,000		0
Full-cost ceiling impairment	251,163,000	$ 0	324,000,000.0	$ 575,164,000	$ 0
Deferred income tax benefit related to impairment charge	$ 62,100,000		$ 80,100,000
Options
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Antidilutive securities (in shares) | shares	2.5	2.5		2.5	2.7
Restricted Stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Antidilutive securities (in shares) | shares	0.7			0.6