Derivative Instruments and Hedging Activities - Effect of Derivative Instruments on the Condensed Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash received (paid) on settlement of derivative instruments, gain (loss) on derivative instruments	$ 5,819	$ 1,204	$ (20,312)	$ (319)
Non-cash gain (loss) on derivative instruments, gain (loss) on derivative instruments	(9,487)	12,601	22,221	47,509
Gain (loss) on derivative instruments	$ (3,668)	$ 13,805	$ 1,909	$ 47,190