Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended				12 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Dec. 31, 2013
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	$ 13,364		$ 32,272		$ 97,751
Restricted cash	895		872		408
Commodity derivatives, net	69,163		57,830	[1]	967	[1]
Convertible preferred securities	16,950	[2]	17,710	[2]	25,720	[2]
Other assets and liabilities	8,799	[1]	8,111	[3]	9,984	[3]
Total	109,171		116,795		134,830
Level 1
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	13,364		32,272		97,751
Restricted cash	895		872		408
Commodity derivatives, net	111,184		79,006	[1]	50,777	[1]
Convertible preferred securities	0	[2]	0	[2]	0	[2]
Other assets and liabilities	10,662	[1]	8,837	[3]	10,143	[3]
Total	136,105		120,987		159,079
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	(42,021)		(21,176)	[1]	(49,810)	[1]
Convertible preferred securities	0	[2]	0	[2]	0	[2]
Other assets and liabilities	(1,863)	[1]	(726)	[3]	(159)	[3]
Total	(43,884)		(21,902)		(49,969)
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		0	[1]	0	[1]
Convertible preferred securities	16,950	[2]	17,710	[2]	25,720	[2]
Other assets and liabilities	0	[1]	0	[3]	0	[3]
Total	16,950		17,710		25,720
Convertible Preferred Stock | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 16,950		$ 17,710		$ 25,720
Market Approach Valuation Technique | Convertible Preferred Stock | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple	7.75		6.60		7.75
Market Approach Valuation Technique | Minimum | Convertible Preferred Stock | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple	7.50		5.50		7.50
Market Approach Valuation Technique | Maximum | Convertible Preferred Stock | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple	8.00		7.00		8.00
Income Approach Valuation Technique | Convertible Preferred Stock | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair Value Inputs, Discount Rate	14.50%		17.00%		14.50%
Income Approach Valuation Technique | Minimum | Convertible Preferred Stock | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair Value Inputs, Discount Rate	14.50%		17.00%		14.50%
Income Approach Valuation Technique | Maximum | Convertible Preferred Stock | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair Value Inputs, Discount Rate	14.50%		17.00%		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)