Investment in Laramie Energy, LLC - Additional Information (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 0			$ 0			$ 46,905,000
Depreciation, depletion, and amortization	$ 22,821,000		$ 22,227,000	66,232,000	$ 65,103,000
Unrealized gain (loss) on derivative contracts				$ 2,733,000	(6,328,000)
Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Ownership interest in Laramie Energy, LLC	46.00%			46.00%
Equity method investments, fair value	$ 0	$ 1,900,000		$ 0			$ 46,905,000
Equity method investment, aggregate cost		47,200,000
Impairment of our investment in Laramie Energy		$ 45,300,000		45,294,000
Equity method investments						$ 0
Depreciation, depletion, and amortization	9,000,000.0		20,700,000	28,300,000	63,100,000
Unrealized gain (loss) on derivative contracts	(5,900,000)		$ (4,300,000)	(7,600,000)	$ 6,800,000
Revolving Credit Facility | Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Credit facility, maximum borrowing amount	400,000,000			400,000,000
Asset borrowing base	200,900,000			200,900,000
Balance outstanding on the revolving credit facility	$ 200,000,000.0			$ 200,000,000.0