Investment in Laramie Energy - Additional Information (Detail) - USD ($)				12 Months Ended
	Oct. 18, 2018	Feb. 28, 2018	Mar. 01, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 29, 2016
Schedule of Equity Method Investments [Line Items]
Investment in Laramie Energy, LLC				$ 0	$ 0	$ 55,000,000
Depreciation, depletion, and amortization				52,642,000	45,989,000	31,617,000
Unrealized gain (loss) on derivative contracts				$ (2,122,000)	989,000	15,479,000
Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Ownership of Laramie Energy, LLC	46.00%	39.10%	42.30%	46.00%			32.40%
Depreciation, depletion, and amortization				$ 66,600,000	50,300,000	42,700,000
Unrealized gain (loss) on derivative contracts				(4,100,000)	46,200,000	$ (34,500,000)
Amount of equity in underlying assets exceeding carrying value				$ 85,200,000	67,200,000
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		$ 20,000,000
Equity method investments, fair value		$ 28,100,000
Shares issued (in shares)		70,227
Additional payments to acquire equity method investments		$ 3,500,000
Piceance Basin | Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Investment in Laramie Energy, LLC			$ 152,100,000
Revolving Credit Facility | Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Line credit maximum borrowing amount				$ 400,000,000
Asset borrowing base currently at				240,000,000
Balance outstanding on the revolving credit facility				$ 210,800,000	$ 171,500,000
Capital Unit, Class A [Member] | Laramie Energy Company
Schedule of Equity Method Investments [Line Items]
Stock repurchased during period (in shares)	138,795
Stock repurchased during period	$ 14,800,000