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Investment in Laramie Energy Additional Information (Detail) - USD ($)
 3 Months Ended 9 Months Ended
Mar. 01, 2016
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]            
Payments to Acquire Equity Method Investments       $ 55,000,000 $ 27,529,000  
Depreciation, depletion and amortization   $ 9,643,000 $ 4,596,000 19,839,000 12,852,000  
Unrealized (loss) gain on derivative contracts       $ 5,804,000 (4,786,000)  
Laramie Energy Company            
Schedule of Equity Method Investments [Line Items]            
Ownership of Laramie Energy, LLC 32.40% 42.30%   42.30%    
Line credit maximum borrowing amount   $ 400,000,000   $ 400,000,000    
Asset borrowing base currently at   170,000,000   170,000,000    
Balance outstanding on the revolving credit facility   114,000,000   114,000,000    
Increase (decrease) in assets of equity method investment capitalized drilling costs obligation paid       55,000,000    
Depreciation, depletion and amortization   13,500,000 7,500,000 34,100,000 23,300,000  
Unrealized (loss) gain on derivative contracts   12,700,000 $ (700,000) (16,800,000) $ (5,100,000)  
Amount of equity in underlying assets exceeding carrying value   $ 70,300,000   $ 70,300,000   $ 55,400,000
Amortization of natural gas and oil properties       15 years    
Piceance Basin | Laramie Energy Company            
Schedule of Equity Method Investments [Line Items]            
Payments to Acquire Equity Method Investments $ 157,500,000