Derivative Commodity Instruments - Quantitative Disclosures About Fair Value (Details) (USD $) In Thousands, unless otherwise specified				3 Months Ended			3 Months Ended				3 Months Ended				3 Months Ended			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Natural Gas Basis - San Juan 2013	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Natural Gas Basis - San Juan 2013 Minimum	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Natural Gas Basis - San Juan 2013 Maximum	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Natural Gas Basis - San Juan 2014	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Natural Gas Basis - San Juan 2014 Minimum	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Natural Gas Basis - San Juan 2014 Maximum	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Natural Gas Basis - Permian 2013	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Natural Gas Basis - Permian 2014	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Natural Gas Basis - Permian 2014 Minimum	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Natural Gas Basis - Permian 2014 Maximum	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Oil Basis - WTS/WTI 2013	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Oil Basis - WTI/WTI 2013	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Oil Basis - WTI/WTI 2013 Minimum	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Oil Basis - WTI/WTI 2013 Maximum	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Natural Gas Liquids 2013	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Natural Gas Liquids 2013 Minimum	Mar. 31, 2013 Income Approach Valuation Technique Level 3 Natural Gas Liquids 2013 Maximum
Fair Value Inputs, Derivatives, Quantitative Information [Line Items]
Fair Value as of March 31, 2013	$ 36,889	$ 105,369	$ 14,529			$ 16,066											$ 8,865
Fair Value as of March 31, 2013									$ (1,854)	$ (2,731)			$ (6,209)	$ (2,207)
Fair Value Inputs, Price Per Cubic Foot				0.14	0.16		0.14	0.16	0.12		0.12	0.14
Fair Value Inputs, Price Per Barrel													0.66		0.18	0.28
Fair Value Inputs, Price Per Gallon																		0.75	0.83