Derivative Financial Instruments Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Derivative settlement (gain) loss	$ 36,665	$ (16,303)	$ 61,193	$ (16,310)
Net derivative (gain) loss	63,749	(55,189)	71,278	(169,963)
Oil Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative settlement (gain) loss	24,430	2,754	45,178	11,838
Net derivative (gain) loss	22,402	(38,194)	36,368	(87,784)
Gas Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative settlement (gain) loss	757	(21,751)	(5,653)	(39,257)
Net derivative (gain) loss	7,000	(6,038)	16,990	(50,506)
NGL Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative settlement (gain) loss	11,478	2,694	21,668	11,109
Net derivative (gain) loss	$ 34,347	$ (10,957)	$ 17,920	$ (31,673)