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Investment in Laramie Energy, LLC - Additional Information (Details) - USD ($)
 3 Months Ended
Feb. 28, 2018
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]        
Depreciation, depletion, and amortization   $ 13,037,000 $ 11,260,000  
Unrealized gain (loss) on derivative contracts   $ 846,000 1,488,000  
Laramie Energy Company        
Schedule of Equity Method Investments [Line Items]        
Ownership interest in Laramie Energy, LLC 39.10% 39.10%   42.30%
Depreciation, depletion, and amortization   $ 14,900,000 13,300,000  
Unrealized gain (loss) on derivative contracts   4,600,000 $ 24,200,000  
Amount of equity in underlying assets exceeding carrying value by   $ 61,500,000   $ 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    
Asset borrowing base   230,000,000    
Balance outstanding on the revolving credit facility   $ 172,500,000    
Unaffiliated Third Party | Laramie Energy Company        
Schedule of Equity Method Investments [Line Items]        
Cash contributions $ 23,500,000      
Equity method investments, fair value $ 28,100,000      
Stock issued during period, new issues (in shares) 70,227