Derivative Instruments and Hedging Activities - Recognized Gain (Loss) on Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Recognized Net Gain (Loss) on Derivative Instruments
Gain (Loss) on Sale of Derivatives	$ (32,539)	$ 4,014	$ 11,308	$ (18,769)
Net gain (loss) on commodity derivatives
Recognized Net Gain (Loss) on Derivative Instruments
Gain (Loss) on Sale of Derivatives	14,800		42,800
Commodity derivatives
Recognized Net Gain (Loss) on Derivative Instruments
Net (losses) gains recognized on derivative instruments	$ (32,500)	$ 4,000	$ 11,300	$ (18,800)