Investment in Laramie Energy, LLC - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	$ 11,284,000	$ 5,100,000	22,544,000	10,196,000
Unrealized gain (loss) on derivative contracts			$ (3,656,000)	5,408,000
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
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	131,000,000		131,000,000
Depreciation, depletion, and amortization	12,500,000	13,000,000	25,800,000	20,600,000
Unrealized gain (loss) on derivative contracts	8,700,000	$ (35,400,000)	32,900,000	$ (29,500,000)
Amount of equity in underlying assets exceeding carrying value by	$ 65,100,000		$ 65,100,000		$ 67,800,000
Amortization of natural gas and oil properties			15 years