Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013		Sep. 30, 2013
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	$ 85,663		$ 97,751		$ 90,093
Restricted cash	173		408		164
Commodity derivatives, net	(126,689)	[1]	967	[1]	7,525	[1]
Convertible preferred securities	15,000	[2]	25,720	[2]	17,710	[2]
Other assets and liabilities	9,035	[3]	9,984	[3]	8,494	[3]
Total	(16,818)		134,830		123,986
Level 1
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	85,663		97,751		90,093
Restricted cash	173		408		164
Commodity derivatives, net	32,606	[1]	50,777	[1]	62,560	[1]
Convertible preferred securities	0	[2]	0	[2]	0	[2]
Other assets and liabilities	10,671	[3]	10,143	[3]	9,539	[3]
Total	129,113		159,079		162,356
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	(159,295)	[1]	(49,810)	[1]	(55,035)	[1]
Convertible preferred securities	0	[2]	0	[2]	0	[2]
Other assets and liabilities	(1,636)	[3]	(159)	[3]	(1,045)	[3]
Total	(160,931)		(49,969)		(56,080)
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	[1]	0	[1]	0	[1]
Convertible preferred securities	15,000	[2]	25,720	[2]	17,710	[2]
Other assets and liabilities	0	[3]	0	[3]	0	[3]
Total	$ 15,000		$ 25,720		$ 17,710

[1] Includes associated cash posted/received as collateral

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

[3] Included in other assets and liabilities are interest rate and foreign currency derivatives and swaptions (Level 2) and deferred compensation assets (Level 1)