Derivative Financial Instruments Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Derivative settlement (gain) loss	$ 24,528	$ (7)
Net derivative (gain) loss	7,529	(114,774)
Oil Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative settlement (gain) loss	20,748	9,084
Net derivative (gain) loss	13,966	(49,590)
Gas Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative settlement (gain) loss	(6,410)	(17,506)
Net derivative (gain) loss	9,990	(44,468)
NGL Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative settlement (gain) loss	10,190	8,415
Net derivative (gain) loss	$ (16,427)	$ (20,716)