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Investment in Laramie Energy, LLC - Additional Information (Details) - USD ($)
 3 Months Ended 6 Months Ended
Feb. 28, 2018
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]            
Depreciation, depletion, and amortization   $ 12,775,000 $ 11,284,000 $ 25,812,000 $ 22,544,000  
Unrealized gain (loss) on derivative contracts       $ (4,878,000) (3,656,000)  
Laramie Energy Company            
Schedule of Equity Method Investments [Line Items]            
Ownership interest in Laramie Energy, LLC 39.10% 39.10%   39.10%   42.30%
Depreciation, depletion, and amortization   $ 17,100,000 12,500,000 $ 32,000,000 25,800,000  
Unrealized gain (loss) on derivative contracts   (8,100,000) $ 8,700,000 (3,500,000) $ 32,900,000  
Amount of equity in underlying assets exceeding carrying value by   61,000,000   $ 61,000,000   $ 67,200,000
Amortization of natural gas and oil properties       15 years    
Revolving Credit Facility | Laramie Energy Company            
Schedule of Equity Method Investments [Line Items]            
Credit facility, maximum borrowing amount   400,000,000   $ 400,000,000    
Asset borrowing base   250,000,000   250,000,000    
Balance outstanding on the revolving credit facility   $ 190,000,000   $ 190,000,000    
Unaffiliated Third Party | Laramie Energy Company            
Schedule of Equity Method Investments [Line Items]            
Cash contributions $ 20,000,000          
Equity method investments, fair value $ 28,100,000          
Stock issued during period, new issues (in shares) 70,227