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, 2012
Natural Gas | San Juan Basin | 2013
Fair Value, Option, Quantitative Disclosures [Line Items]
Net derivative asset (liability), at fair value (in USD)	$ 38,254
Natural Gas | San Juan Basin | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Net derivative asset (liability), at fair value (in USD)	21,100
Natural Gas | Permian Basin | 2013
Fair Value, Option, Quantitative Disclosures [Line Items]
Net derivative asset (liability), at fair value (in USD)	0
Natural Gas | Permian Basin | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Net derivative asset (liability), at fair value (in USD)	(1)
Crude Oil | WTS/WTI Basis Swaps | 2013
Fair Value, Option, Quantitative Disclosures [Line Items]
Net derivative asset (liability), at fair value (in USD)	10,338
Crude Oil | WTI/WTI Basis Swaps | 2013
Fair Value, Option, Quantitative Disclosures [Line Items]
Net derivative asset (liability), at fair value (in USD)	8,217
Natural Gas Liquids | Liquids Swaps | 2013
Fair Value, Option, Quantitative Disclosures [Line Items]
Net derivative asset (liability), at fair value (in USD)	$ 11,821
Discounted Cash Flow Valuation Technique [Member] | Minimum | Natural Gas | San Juan Basin | 2013
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per Mcf)	(0.15)	[1]
Discounted Cash Flow Valuation Technique [Member] | Minimum | Natural Gas | San Juan Basin | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per Mcf)	(0.13)	[1]
Discounted Cash Flow Valuation Technique [Member] | Minimum | Natural Gas | Permian Basin | 2013
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per Mcf)	(0.13)	[1]
Discounted Cash Flow Valuation Technique [Member] | Minimum | Natural Gas | Permian Basin | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per Mcf)	(0.12)	[1]
Discounted Cash Flow Valuation Technique [Member] | Minimum | Crude Oil | WTS/WTI Basis Swaps | 2013
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per bbl)	(5.19)	[1]
Discounted Cash Flow Valuation Technique [Member] | Minimum | Crude Oil | WTI/WTI Basis Swaps | 2013
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per bbl)	(2.92)	[1]
Discounted Cash Flow Valuation Technique [Member] | Minimum | Natural Gas Liquids | Liquids Swaps | 2013
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per gal)	0.73	[1]
Discounted Cash Flow Valuation Technique [Member] | Maximum | Natural Gas | San Juan Basin | 2013
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per Mcf)	(0.16)	[1]
Discounted Cash Flow Valuation Technique [Member] | Maximum | Natural Gas | San Juan Basin | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per Mcf)	(0.17)	[1]
Discounted Cash Flow Valuation Technique [Member] | Maximum | Natural Gas | Permian Basin | 2014
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per Mcf)	(0.13)	[1]
Discounted Cash Flow Valuation Technique [Member] | Maximum | Crude Oil | WTI/WTI Basis Swaps | 2013
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per bbl)	(3.62)	[1]
Discounted Cash Flow Valuation Technique [Member] | Maximum | Natural Gas Liquids | Liquids Swaps | 2013
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value inputs, derivative, nonmonetary notional amount (USD per gal)	0.82	[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.