Investment in Piceance Energy Additional Information (Detail) - USD ($)			3 Months Ended		6 Months Ended
	May. 29, 2015	Mar. 09, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	May. 28, 2015	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Cash capital contribution					$ 27,529,000	$ 0
Depreciation, depletion, and amortization			$ 5,005,000	$ 3,290,000	8,256,000	6,351,000
Unrealized loss on derivative contracts					$ (426,000)	(191,000)
Amortization of natural gas and oil properties					15 years
Piceance Energy [Member]
Schedule of Equity Method Investments [Line Items]
Ownership of Piceance Energy, LLC	34.00%		34.00%		34.00%		33.34%
Line credit maximum borrowing amount			$ 400,000,000		$ 400,000,000
Asset borrowing base currently at			110,000,000		110,000,000
Balance outstanding on the revolving credit facility			48,500,000		48,500,000
Cash capital contribution	$ 13,800,000	$ 13,800,000
Depreciation, depletion, and amortization			8,400,000	8,600,000	15,800,000	15,300,000
Unrealized loss on derivative contracts			(3,700,000)	$ (1,700,000)	(4,400,000)	$ (157,000)
Amount of equity in underlying assets exceeding carrying value			$ 14,300,000		$ 14,300,000			$ 14,700,000