Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011		Jun. 30, 2011
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	$ 104		$ 183		$ 182
Restricted cash	5,644		18,651		12,572
Commodity derivatives, net	96,636		68,846		169,859
Convertible preferred securities	17,350	[1]	20,360	[1]	15,790	[1]
Other assets and liabilities	5,160	[2]	4,046	[2]	4,462	[2]
Total	124,894		112,086		202,865
Level 1
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	104		183		182
Restricted cash	5,644		18,651		12,572
Commodity derivatives, net	48,558		43,503		89,769
Convertible preferred securities	0	[1]	0	[1]	0	[1]
Other assets and liabilities	7,182	[2]	6,224	[2]	6,345	[2]
Total	61,488		68,561		108,868
Level 2
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	0		0		0
Restricted cash	0		0		0
Commodity derivatives, net	48,078		22,876		71,296
Convertible preferred securities	0	[1]	0	[1]	0	[1]
Other assets and liabilities	(2,022)	[2]	0	[2]	0	[2]
Total	46,056		22,876		71,296
Level 3
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	0		0		0
Restricted cash	0		0		0
Commodity derivatives, net	0		2,467		8,794
Convertible preferred securities	17,350	[1]	20,360	[1]	15,790	[1]
Other assets and liabilities	0	[2]	(2,178)	[2]	(1,883)	[2]
Total	$ 17,350		$ 20,649		$ 22,701

[1] Recorded in “Other noncurrent assets” on the Company’s Consolidated Balance Sheets

[2] Included in other assets and liabilities is interest rate and foreign currency derivatives, swaptions and deferred compensation assets.