Quarterly report pursuant to Section 13 or 15(d)

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

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Derivative Financial Instruments -Changes in Fair Value of Commodity Derivative Contracts Recognized in Earnings (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
Derivative (gain) loss:        
Realized $ 9,640 $ (1,961) $ 14,310 $ 2,310
Unrealized 3,439 (10,879) 6,261 (11,783)
Total $ 13,079 $ (12,840) $ 20,571 $ (9,473)