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Derivatives - Narrative (Details)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2015
USD ($)
Jun. 30, 2014
USD ($)
Jun. 30, 2015
USD ($)
Institution
Jun. 30, 2014
USD ($)
Derivative Instruments and Hedging Activities Disclosure        
Number of counterparties to open derivative contracts | Institution     9  
Loss (gain) on derivative contracts $ 33,004 $ 85,292 $ (16,823) $ 127,783
Commodity Derivatives        
Derivative Instruments and Hedging Activities Disclosure        
Loss (gain) on derivative contracts 33,000 85,300 (16,800) 127,800
Commodity Derivatives | Cash        
Derivative Instruments and Hedging Activities Disclosure        
Loss (gain) on derivative contracts $ (74,400) 13,100 $ (211,300) $ 96,400
Early Settlements | Commodity Derivatives | Cash        
Derivative Instruments and Hedging Activities Disclosure        
Loss (gain) on derivative contracts   $ 69,600    
Lenders of Senior Credit Facility        
Derivative Instruments and Hedging Activities Disclosure        
Number of counterparties to open derivative contracts | Institution     5