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10. Derivative Instruments (Tables)
3 Months Ended
Mar. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]  
Realized and unrealized gains and losses on derivative instruments
    Three Months
Ended
 
    March 31, 2016  
Realized gain (loss) on derivatives:      
Crude oil fixed price swaps   $  
Crude oil collars      
Realized loss on derivatives, net   $  
         
Gain (loss) on the mark-to-market of derivatives:        
Crude oil fixed price swaps   $ (19,898 )
Crude oil collars     4,011  
Gain (loss) on the mark-to-market of derivatives, net   $ (15,887 )