Derivative Financial Instruments Gains and Losses (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Derivative settlement gain (loss)		$ (57,496)	$ (113,695)	$ (306,234)	$ (387,719)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		(28,037)	(212,253)	121,086	(285,491)
NYMEX Oil Contracts [Member]
Gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Derivative settlement gain (loss)		(49,241)	(90,493)	(221,397)	(270,622)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		(733)	(131,728)	49,608	(138,839)
Gas Contracts [Member]
Gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Gain on Early Settlement of Derivatives					(15,300)
Derivative settlement gain (loss)	[1]	(10,096)	(19,167)	(82,588)	(92,279)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		(14,006)	(66,538)	24,460	(142,807)
NGL Contracts [Member]
Gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Derivative settlement gain (loss)		1,841	(4,035)	(2,249)	(24,818)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		$ (13,298)	$ (13,987)	$ 47,018	$ (3,845)

[1] Natural gas derivative settlements for the nine months ended September 30, 2015, include a $15.3 million gain on the early settlement of future contracts as a result of divesting the Company’s Mid-Continent assets during the second quarter of 2015.