Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	$ 10,623	$ 183	$ 10,597
Restricted cash	18,785	18,651	12,353
Short term investments	19,996
Commodity derivatives, net	(1,431)	68,846	123,784
Convertible preferred securities	20,360	20,360	15,790
Other assets and liabilities	5,126	4,046	4,789
Total	73,459	112,086	167,313
Level 1 [Member]
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	10,623	183	10,597
Restricted cash	18,785	18,651	12,353
Short term investments	19,996
Commodity derivatives, net	(12,280)	43,503	(13,486)
Other assets and liabilities	7,211	6,224	6,291
Total	44,335	68,561	15,755
Level 2 [Member]
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives, net	10,849	22,876	122,287
Other assets and liabilities	(2,085)
Total	8,764	22,876	122,287
Level 3 [Member]
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives, net		2,467	14,983
Convertible preferred securities	20,360	20,360	15,790
Other assets and liabilities		(2,178)	(1,502)
Total	$ 20,360	$ 20,649	$ 29,271