Annual report pursuant to Section 13 and 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
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Gain Loss On Derivative Instruments Net Pretax [Abstract]      
Derivative (gain) loss $ (4,199) $ 2,926 $ (14,375)