Investment in Laramie Energy, LLC - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Investment in Laramie Energy, LLC	$ 137,448,000		$ 137,448,000		$ 136,656,000
Depreciation, depletion, and amortization	$ 21,919,000	$ 12,775,000	42,876,000	$ 25,812,000
Unrealized gain (loss) on derivative contracts			$ (21,086,000)	(4,878,000)
Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Ownership interest in Laramie Energy, LLC	46.00%		46.00%
Investment in Laramie Energy, LLC	$ 137,448,000		$ 137,448,000		136,656,000
Investment valuation, historical average period	5 years
Depreciation, depletion, and amortization	$ 21,000,000	17,100,000	42,400,000	32,000,000
Unrealized gain (loss) on derivative contracts	8,400,000	$ (8,100,000)	11,100,000	$ (3,500,000)
Amount of equity in underlying assets exceeding carrying value by	81,900,000		$ 81,900,000		$ 85,200,000
Amortization of natural gas and oil properties, term			15 years
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	240,000,000		240,000,000
Balance outstanding on the revolving credit facility	201,000,000		201,000,000
Minimum | Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Fair value of investment	100,000,000		100,000,000
Maximum | Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Fair value of investment	$ 123,000,000		$ 123,000,000