Investment in Laramie Energy - Additional Information (Detail) - USD ($)				12 Months Ended
	Jul. 31, 2015	May 29, 2015	Mar. 09, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Mar. 01, 2016	Feb. 29, 2016	Jul. 30, 2015	May 28, 2015
Schedule of Equity Method Investments [Line Items]
Investment in Laramie Energy, LLC				$ 55,000,000	$ 27,529,000	$ 12,000
Depreciation, depletion, amortization and accretion				31,617,000	19,918,000	14,897,000
Unrealized loss (gain) on derivative contracts				$ 15,479,000	(10,896,000)	1,015,000
Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Ownership of Laramie Energy, LLC	32.40%	34.00%		42.30%			42.30%	32.40%	34.00%	33.34%
Line credit maximum borrowing amount				$ 400,000,000
Asset borrowing base currently at				170,000,000
Balance outstanding on the revolving credit facility				117,500,000	77,300,000
Investment in Laramie Energy, LLC		$ 13,800,000	$ 13,800,000
Other than temporary impairment charge				0	(41,081,000)	0
Aggregate cash purchase price				55,000,000	27,529,000	12,000
Depreciation, depletion, amortization and accretion				42,700,000	24,600,000	32,800,000
Unrealized loss (gain) on derivative contracts				(34,500,000)	(16,600,000)	$ 9,800,000
Impairment of unproved properties					12,300,000
Amount of equity in underlying assets exceeding carrying value				$ 67,800,000	$ 55,400,000
Amortization of natural gas and oil properties				15 years
Unaffiliated Third Party | Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Investment in Laramie Energy, LLC	$ 19,000,000
Piceance Basin | Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Investment in Laramie Energy, LLC				$ 157,500,000