Quarterly report pursuant to Section 13 or 15(d)

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

v3.8.0.1
Derivative Financial Instruments - Changes in Fair Value and Settlements of Commodity Derivative Contracts (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Gain Loss On Derivative Instruments Net Pretax [Abstract]        
Derivative (gain) loss $ 2,879 $ 412 $ (4,765) $ 2,861