Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	$ 78,674		$ 183
Restricted cash	398		18,651
Commodity derivatives, net	70,600		68,846
Convertible preferred securities	17,200	[1]	20,360	[1]
Other assets and liabilities	5,704	[2]	4,046	[2]
Total	172,576		112,086
Level 1
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	78,674		183
Restricted cash	398		18,651
Commodity derivatives, net	46,966		43,503
Convertible preferred securities	0	[1]	0	[1]
Other assets and liabilities	7,813	[2]	6,224	[2]
Total	133,851		68,561
Level 2
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	0		0
Restricted cash	0		0
Commodity derivatives, net	23,634		22,876
Convertible preferred securities	0	[1]	0	[1]
Other assets and liabilities	(2,109)	[2]	0	[2]
Total	21,525		22,876
Level 3
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	0		0
Restricted cash	0		0
Commodity derivatives, net	0		2,467
Convertible preferred securities	17,200	[1]	20,360	[1]
Other assets and liabilities	0	[2]	(2,178)	[2]
Total	$ 17,200		$ 20,649

[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.