Derivative Instruments and Hedging Activities - Effect of Derivative Instruments on the Condensed Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash received (paid) on settlement of derivative instruments, gain (loss) on derivative instruments	$ (41)	$ 3,906	$ (20,354)	$ 3,587
Non-cash gain (loss) on derivative instruments, gain (loss) on derivative instruments	(3,496)	(4,742)	18,726	42,766
Gain (loss) on derivative instruments	$ (3,537)	$ (836)	$ (1,628)	$ 46,353