Investment in Laramie Energy Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 01, 2016	Dec. 31, 2015	Jul. 31, 2015
Schedule of Equity Method Investments [Line Items]
Depreciation, depletion and amortization	$ 5,095,000	$ 3,251,000
Unrealized loss (gain) on derivative contracts	(1,609,000)	(2,406,000)
Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Line credit maximum borrowing amount	400,000,000
Asset borrowing base currently at	170,000,000
Balance outstanding on the revolving credit facility	$ 131,000,000
Aggregate cost			$ 55,000,000
Ownership of Laramie Energy, LLC	42.30%		42.30%		32.40%
Depreciation, depletion and amortization	$ 7,600,000	7,400,000
Unrealized loss (gain) on derivative contracts	(5,900,000)	$ 700,000
Amount of equity in underlying assets exceeding carrying value	$ 73,300,000			$ 55,400,000
Amortization of natural gas and oil properties	15 years
Piceance Basin | Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Aggregate cost			$ 157,500,000