Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	$ 183	$ 213
Commodity derivatives, net	68,846	203,688
Convertible preferred securities	20,360	15,790
Other assets and liabilities	22,697	15,827
Total	112,086	235,518
Level 1 [Member]
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	183	213
Commodity derivatives, net	43,503	61,559
Convertible preferred securities	0	0
Other assets and liabilities	24,875	17,983
Total	68,561	79,755
Level 2 [Member]
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	0	0
Commodity derivatives, net	22,876	129,723
Convertible preferred securities	0	0
Other assets and liabilities	0	0
Total	22,876	129,723
Level 3 [Member]
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	0	0
Commodity derivatives, net	2,467	12,406
Convertible preferred securities	20,360	15,790
Other assets and liabilities	(2,178)	(2,156)
Total	$ 20,649	$ 26,040