Investment in Laramie Energy, LLC - Additional Information (Details) - USD ($)		3 Months Ended		9 Months Ended
	Feb. 28, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Depreciation, depletion, and amortization		$ 13,192,000	$ 11,304,000	$ 39,004,000	$ 33,848,000
Unrealized gain (loss) on derivative contracts				$ (8,105,000)	(557,000)
Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Ownership interest in Laramie Energy, LLC	39.10%	39.10%		39.10%		42.30%
Depreciation, depletion, and amortization		$ 20,700,000	12,300,000	$ 52,700,000	38,100,000
Unrealized gain (loss) on derivative contracts		(3,200,000)	$ 2,300,000	(6,700,000)	$ 35,200,000
Amount of equity in underlying assets exceeding carrying value by		60,000,000		$ 60,000,000		$ 67,200,000
Amortization of natural gas and oil properties				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		250,000,000		250,000,000
Balance outstanding on the revolving credit facility		$ 193,500,000		$ 193,500,000
Unaffiliated Third Party | Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Cash contributions	$ 20,000,000
Equity method investments, fair value	$ 28,100,000
Stock issued during period, new issues (in shares)	70,227
Additional cash payment	$ 3,500,000