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Investment in Laramie Energy - Additional Information (Detail) - USD ($)
 12 Months Ended
Mar. 01, 2016
Jul. 31, 2015
May 29, 2015
Mar. 09, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Feb. 29, 2016
Jul. 30, 2015
May 28, 2015
Schedule of Equity Method Investments [Line Items]                    
Investment in Laramie Energy, LLC         $ 0 $ 55,000,000 $ 27,529,000      
Depreciation, depletion, and amortization         45,989,000 31,617,000 19,918,000      
Unrealized gain (loss) on derivative contracts         $ 989,000 15,479,000 (10,896,000)      
Laramie Energy Company                    
Schedule of Equity Method Investments [Line Items]                    
Ownership of Laramie Energy, LLC 42.30% 32.40% 34.00%   42.30%     32.40% 34.00% 33.34%
Investment in Laramie Energy, LLC     $ 13,800,000 $ 13,800,000            
Other than temporary impairment charge         $ 0 0 (41,081,000)      
Aggregate cash purchase price $ 55,000,000       0 55,000,000 27,529,000      
Depreciation, depletion, and amortization         50,300,000 42,700,000 24,600,000      
Unrealized gain (loss) on derivative contracts         46,200,000 (34,500,000) 16,600,000      
Impairment of unproved properties             $ 12,300,000      
Amount of equity in underlying assets exceeding carrying value         $ 67,200,000 69,900,000        
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 $ 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         230,000,000          
Balance outstanding on the revolving credit facility         $ 171,500,000 $ 117,500,000