Derivative Financial Instruments Gains and Losses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Unrealized and realized gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Gain (Loss) on Hedging Activity	$ 99	$ 1,652
Cash settlement (gain) and loss	(11,792)	(5,436)
Unrealized Gain (Loss) on Derivatives	42,364	7,652
Unrealized and Realized Derivative loss	30,572	2,216
Amount of (gain) loss reclassified from AOCL to realized hedge gain (loss)	61	(1,034)
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	277	8,299
Unrealized Gain (Loss) on Derivatives	3,789	29,491
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	(9,824)	(15,212)
Unrealized Gain (Loss) on Derivatives	40,069	(17,634)
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	(2,245)	1,477
Unrealized Gain (Loss) on Derivatives	$ (1,494)	$ (4,205)