Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011		Sep. 30, 2011
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	$ 39,904		$ 183		$ 21,481
Restricted cash	160		18,651		11,920
Commodity derivatives, net	129,132		68,846		84,660
Convertible preferred securities	17,350	[1]	20,360	[1]	15,790	[1]
Other assets and liabilities	5,549	[2]	4,046	[2]	3,510	[2]
Total	192,095		112,086		137,361
Level 1
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	39,904		183		21,481
Restricted cash	160		18,651		11,920
Commodity derivatives, net	15,619		43,503		84,365
Convertible preferred securities	0	[1]	0	[1]	0	[1]
Other assets and liabilities	7,515	[2]	6,224	[2]	5,748	[2]
Total	63,198		68,561		123,514
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	113,513		22,876		(2,988)
Convertible preferred securities	0	[1]	0	[1]	0	[1]
Other assets and liabilities	(1,966)	[2]	0	[2]	0	[2]
Total	111,547		22,876		(2,988)
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		3,283
Convertible preferred securities	17,350	[1]	20,360	[1]	15,790	[1]
Other assets and liabilities	0	[2]	(2,178)	[2]	(2,238)	[2]
Total	$ 17,350		$ 20,649		$ 16,835

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

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