Derivative Instruments and Hedging Activities - Recognized Gain (Loss) on Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Recognized Net Gain (Loss) on Derivative Instruments
Gain (Loss) on Sale of Derivatives	$ 21,527	$ (40,002)	$ 43,847	$ (22,783)
Commodity Derivative Instruments
Recognized Net Gain (Loss) on Derivative Instruments
Net (losses) gains recognized on derivative instruments	$ 21,500	$ (40,000)	$ 43,800	$ (22,800)