Derivative Instruments and Hedging Activities (Gain (Loss) Reclassified from Accumulated OCI into Income) (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ (12,451)	$ 9,194	$ (19,256)	$ 9,577
Oil and Natural Gas Commodity Contract [Member] | Oil and Gas Revenue [Member]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(11,860)	9,663	(18,185)	10,464
Interest Rate Swap [Member] | Net Interest Expense and Other [Member]
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ (591)	$ (469)	$ (1,071)	$ (887)