Derivative Financial Instruments Gains and Losses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Derivative cash settlement gain	$ (1,288)	[1]	$ 10,921	[1]	$ (12,715)	[1]	$ (32,803)	[1]
Derivative (gain) loss	39,933	[2]	55,856	[2]	(14,685)	[2]	(40,040)	[2]
Amount of (gain) loss reclassified from AOCL to realized hedge gain (loss)	308		(315)		1,115		(1,465)
Realized hedge gain (loss)	(489)		501		(1,777)		2,338
Crude oil
Gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Derivative cash settlement gain	(13,538)		(2,472)		(13,786)		(13,142)
Derivative (gain) loss	30,488		30,667		8,233		(32,616)
Natural Gas
Gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Derivative cash settlement gain	11,019		9,031		18,752		40,495
Derivative (gain) loss	(2,264)		28,231		(12,462)		40,464
Natural Gas Liquids
Gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Derivative cash settlement gain	1,231		4,362		7,749		5,450
Derivative (gain) loss	$ 10,421		$ 7,879		$ 2,259		$ (15,085)

[1] (1) Derivative cash settlement (gain) loss is reported in the derivative cash settlement gain line item on the condensed consolidated statements of cash flows within net cash provided by operating activities.

[2] (2) Total derivative (gain) loss is reported in the derivative gain line item on the condensed consolidated statements of cash flows within net cash provided by operating activities.