Derivative Financial Instruments Gains and Losses (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative settlement (gain) loss		$ (21,234)	$ (329,478)	$ (512,566)
Net derivative (gain) loss		26,414	250,633	(408,831)
Oil Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative settlement (gain) loss		31,176	(243,102)	(362,219)
Net derivative (gain) loss		71,502	85,370	(191,165)
Gas Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative settlement (gain) loss	[1]	(87,857)	(94,936)	(123,180)
Net derivative (gain) loss		(76,315)	81,060	(189,734)
Gain on Early Settlement of Derivatives				15,300
NGL Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative settlement (gain) loss		35,447	8,560	(27,167)
Net derivative (gain) loss		$ 31,227	$ 84,203	$ (27,932)
Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Number of Instruments Held		0	0	0

[1] Gas derivative settlements for the year ended December 31, 2015, include $15.3 million of early settlements of futures contracts as a result of divesting assets in the Company’s Mid-Continent region.