Derivative Instruments - Gain (Loss) on Derivative Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Gain due to changes in fair value of derivative instruments	$ (123,479)	$ 213,679	$ (174,506)
Natural gas derivatives
Derivative [Line Items]
Gain due to changes in fair value of derivative instruments	(116,130)	232,143	(165,933)
Oil derivatives
Derivative [Line Items]
Gain due to changes in fair value of derivative instruments	(13,084)	(3,350)	(5,387)
Natural gas liquids derivatives
Derivative [Line Items]
Gain due to changes in fair value of derivative instruments	$ 5,735	$ (15,114)	$ (3,186)