Financial Instruments and Risk Management (Level 3 Fair Value Measurements of Derivative Commodity Instruments) (Details) (Level 3, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013
Natural Gas | San Juan Basin | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Net derivative asset (liability), at fair value (in USD)	$ 18,159
Natural Gas | San Juan Basin | 2015
Fair Value, Option, Quantitative Disclosures [Line Items]
Net derivative asset (liability), at fair value (in USD)	1,056
Natural Gas | Permian Basin | 2015
Fair Value, Option, Quantitative Disclosures [Line Items]
Net derivative asset (liability), at fair value (in USD)	(1,948)
Natural Gas Liquids | Liquids Swaps | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Net derivative asset (liability), at fair value (in USD)	$ 1,022
Discounted Cash Flow Valuation Technique | Minimum | Natural Gas | San Juan Basin | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per Mcf)	0.17	[1]
Discounted Cash Flow Valuation Technique | Minimum | Natural Gas | San Juan Basin | 2015
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per Mcf)	0.26	[1]
Discounted Cash Flow Valuation Technique | Minimum | Natural Gas | Permian Basin | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per Mcf)	0.18	[1]
Discounted Cash Flow Valuation Technique | Minimum | Natural Gas Liquids | Liquids Swaps | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per gal)	0.80	[1]
Discounted Cash Flow Valuation Technique | Maximum | Natural Gas | San Juan Basin | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per Mcf)	0.20	[1]
Discounted Cash Flow Valuation Technique | Maximum | Natural Gas | San Juan Basin | 2015
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per Mcf)	0.26	[1]
Discounted Cash Flow Valuation Technique | Maximum | Natural Gas | Permian Basin | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per Mcf)	0.20	[1]
Discounted Cash Flow Valuation Technique | Maximum | Natural Gas Liquids | Liquids Swaps | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per gal)	0.81	[1]

[1] Discounted cash flow represents an income approach in calculating fair value including the referenced unobservable input and a discount reflecting credit quality of the counterparty.