Fair Value Measurements (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	$ 21,481	$ 213	$ 252
Commodity derivatives, net	84,660	203,688	137,047
Convertible preferred securities	15,790	15,790	13,100
Other assets and liabilities	15,430	15,827	5,700
Total	137,361	235,518	156,099
Level 1 [Member]
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	21,481	213	252
Commodity derivatives, net	84,365	61,559	78,345
Convertible preferred securities	0	0	0
Other assets and liabilities	17,668	17,983	8,315
Total	123,514	79,755	86,912
Level 2 [Member]
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	0	0	0
Commodity derivatives, net	(2,988)	129,723	57,480
Convertible preferred securities	0	0	0
Other assets and liabilities	0	0	0
Total	(2,988)	129,723	57,480
Level 3 [Member]
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	0	0	0
Commodity derivatives, net	3,283	12,406	1,222
Convertible preferred securities	15,790	15,790	13,100
Other assets and liabilities	(2,238)	(2,156)	(2,615)
Total	$ 16,835	$ 26,040	$ 11,707