Fair Value Measurements (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010
Assets and liabilities measured at fair value on a recurring basis
Cash and cash equivalents	$ 182	$ 213	$ 173,797
Commodity derivatives, net	169,859	203,688	(43,373)
Convertible preferred securities	15,790	15,790	13,100
Other assets and liabilities	17,034	15,827	6,309
Total	202,865	235,518	149,833
Level 1 [Member]
Assets and liabilities measured at fair value on a recurring basis
Cash and cash equivalents	182	213	173,797
Commodity derivatives, net	89,769	61,559	(20,240)
Convertible preferred securities	0	0	0
Other assets and liabilities	18,917	17,983	8,586
Total	108,868	79,755	162,143
Level 2 [Member]
Assets and liabilities measured at fair value on a recurring basis
Cash and cash equivalents	0	0	0
Commodity derivatives, net	71,296	129,723	(23,140)
Convertible preferred securities	0	0	0
Other assets and liabilities	0	0	0
Total	71,296	129,723	(23,140)
Level 3 [Member]
Assets and liabilities measured at fair value on a recurring basis
Cash and cash equivalents	0	0	0
Commodity derivatives, net	8,794	12,406	7
Convertible preferred securities	15,790	15,790	13,100
Other assets and liabilities	(1,883)	(2,156)	(2,277)
Total	$ 22,701	$ 26,040	$ 10,830