Derivative Financial Instruments Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, (Gain) Loss [Line Items]
Derivative settlement (gain) loss	$ 40,718	$ (13,092)	$ 101,911	$ (29,402)
Net derivative (gain) loss	178,026	80,599	249,304	(89,364)
Oil Contracts [Member]
Derivative Instruments, (Gain) Loss [Line Items]
Derivative settlement (gain) loss	16,798	2,472	61,976	14,310
Net derivative (gain) loss	110,413	45,874	146,781	(41,910)
Gas Contracts [Member]
Derivative Instruments, (Gain) Loss [Line Items]
Derivative settlement (gain) loss	802	(24,088)	(4,851)	(63,345)
Net derivative (gain) loss	4,309	(6,068)	21,299	(56,574)
NGL Contracts [Member]
Derivative Instruments, (Gain) Loss [Line Items]
Derivative settlement (gain) loss	23,118	8,524	44,786	19,633
Net derivative (gain) loss	$ 63,304	$ 40,793	$ 81,224	$ 9,120