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Derivatives - Narrative (Details)
3 Months Ended
Mar. 31, 2016
USD ($)
institution
Mar. 31, 2015
USD ($)
Dec. 31, 2015
Derivative Instruments and Hedging Activities Disclosure      
Gain on derivative contracts $ 2,808,000 $ 49,827,000  
Number of counterparties to open derivative contracts | institution 6    
Senior Notes | 8.75% Senior Notes due 2020      
Derivative Instruments and Hedging Activities Disclosure      
Long-term debt, fixed interest rate 8.75%   8.75%
Aggregate outstanding principal amount $ 100,000,000    
Lenders of Senior Credit Facility      
Derivative Instruments and Hedging Activities Disclosure      
Number of counterparties to open derivative contracts | institution 3    
Commodity Derivatives      
Derivative Instruments and Hedging Activities Disclosure      
Gain on derivative contracts $ 2,800,000 49,800,000  
Commodity Derivatives | Cash      
Derivative Instruments and Hedging Activities Disclosure      
Gain on derivative contracts $ 25,500,000 $ 137,000,000