Quarterly report pursuant to Section 13 or 15(d)

Derivative Financial Instruments - Changes in Fair Value of Commodity Derivative Contracts (Details)

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Derivative Financial Instruments - Changes in Fair Value of Commodity Derivative Contracts (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Gain Loss On Derivative Instruments Net Pretax [Abstract]        
Derivative loss $ 1,078 $ 13,079 $ 1,078 $ 20,571