Derivative Financial Instruments Gains and Losses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Unrealized and realized gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Realized hedge gain (loss)	$ (1,189)	$ 185	$ (1,288)	$ 1,837
Cash settlement (gain) and loss	(2,211)	(16,446)	(14,003)	(21,882)
Unrealized Gain (Loss) on Derivatives	(82,979)	(81,666)	(40,615)	(74,014)
Unrealized and realized derivative gain	(85,190)	(98,112)	(54,618)	(95,896)
Amount of (gain) loss reclassified from AOCL to realized hedge gain (loss)	746	(116)	807	(1,150)
Crude oil
Unrealized and realized gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Cash settlement (gain) and loss	(29)	2,371	248	10,670
Unrealized Gain (Loss) on Derivatives	(26,044)	(92,774)	(22,255)	(63,283)
Natural Gas
Unrealized and realized gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Cash settlement (gain) and loss	2,091	(16,252)	(7,733)	(31,464)
Unrealized Gain (Loss) on Derivatives	(50,267)	29,867	(10,198)	12,233
Natural Gas Liquids
Unrealized and realized gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Cash settlement (gain) and loss	(4,273)	(2,565)	(6,518)	(1,088)
Unrealized Gain (Loss) on Derivatives	$ (6,668)	$ (18,759)	$ (8,162)	$ (22,964)