Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified																										3 Months Ended
	Mar. 31, 2014		Dec. 31, 2013		Mar. 31, 2013		Mar. 31, 2014 Level 1		Dec. 31, 2013 Level 1		Mar. 31, 2013 Level 1		Mar. 31, 2014 Level 2		Dec. 31, 2013 Level 2		Mar. 31, 2013 Level 2		Mar. 31, 2014 Level 3		Dec. 31, 2013 Level 3		Mar. 31, 2013 Level 3		Mar. 31, 2014 Convertible Preferred Stock Level 3	Mar. 31, 2014 Market Approach Valuation Technique Convertible Preferred Stock Level 3	Mar. 31, 2014 Market Approach Valuation Technique Minimum Convertible Preferred Stock Level 3	Mar. 31, 2014 Market Approach Valuation Technique Maximum Convertible Preferred Stock Level 3	Mar. 31, 2014 Income Approach Valuation Technique Convertible Preferred Stock Level 3	Mar. 31, 2014 Income Approach Valuation Technique Minimum Convertible Preferred Stock Level 3	Mar. 31, 2014 Income Approach Valuation Technique Maximum Convertible Preferred Stock Level 3
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	$ 25,821		$ 97,751		$ 49,202		$ 25,821		$ 97,751		$ 49,202		$ 0		$ 0		$ 0		$ 0		$ 0		$ 0
Restricted cash	652		408		635		652		408		635		0		0		0		0		0		0
Commodity derivatives, net	87,808		967	[1]	105,515	[1]	82,626		50,777	[1]	110,581	[1]	5,182		(49,810)	[1]	(5,066)	[1]	0		0	[1]	0	[1]
Convertible preferred securities	20,530	[2]	25,720	[2]	17,710	[2]	0	[2]	0	[2]	0	[2]	0	[2]	0	[2]	0	[2]	20,530	[2]	25,720	[2]	17,710	[2]
Other assets and liabilities	10,009	[1]	9,984	[3]	7,077	[3]	10,960	[1]	10,143	[3]	8,861	[3]	(951)	[1]	(159)	[3]	(1,784)	[3]	0	[1]	0	[3]	0	[3]
Total	144,820		134,830		180,139		120,059		159,079		169,279		4,231		(49,969)		(6,850)		20,530		25,720		17,710
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value																									$ 20,530
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple																										7.75	7.50	8.00
Fair Value Inputs, Discount Rate																													14.50%	14.50%	14.50%

[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)