Investment in Laramie Energy, LLC - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Cash contributions			$ 0	$ 55,000,000
Depreciation, depletion, and amortization	$ 11,304,000	$ 9,643,000	33,848,000	19,839,000
Unrealized gain (loss) on derivative contracts			$ (557,000)	5,804,000
Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Ownership interest in Laramie Energy, LLC	42.30%		42.30%
Cash contributions					$ 55,000,000	$ 27,500,000
Depreciation, depletion, and amortization	$ 12,300,000	13,500,000	$ 38,100,000	34,100,000
Unrealized gain (loss) on derivative contracts	2,300,000	$ 12,700,000	35,200,000	$ (16,800,000)
Amount of equity in underlying assets exceeding carrying value by	63,800,000		$ 63,800,000		$ 67,800,000
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	210,000,000		210,000,000
Balance outstanding on the revolving credit facility	$ 149,000,000		$ 149,000,000