Derivative Instruments and Hedging Activities (Derivatives by balance sheet location) (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative Assets, Current	$ 19,477	$ 47,402
Derivative Assets, Noncurrent	36	29
Derivative Liabilities, Current	(2,842)	(26,523)
Derivative Liabilities, Noncurrent	(23,781)	(18,872)
Total derivatives not designated as hedging instruments	(7,110)	2,036
Crude Oil Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets, Current	19,477	23,452
Derivative Assets, Noncurrent	36	29
Derivative Liabilities, Current	(2,659)	(22,610)
Derivative Liabilities, Noncurrent	(23,781)	(18,702)
Natural Gas Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets, Current	0	23,950
Deferred Premiums [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities, Current	(183)	(3,913)
Derivative Liabilities, Noncurrent	$ 0	$ (170)

Carrying amount as of the balance sheet date of the asset arising from commodity contracts such as futures contracts tied to the movement of a particular commodity, which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.

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Carrying amount as of the balance sheet date of the asset arising from commodity contracts such as futures contracts tied to the movement of a particular commodity, which are expected to be converted into cash or otherwise disposed of after a year or beyond the normal operating cycle, if longer.

+ References

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Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 107

-Paragraph 10

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

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-Name Statement of Financial Accounting Standard (FAS)

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-Paragraph 4, 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Carrying amount as of the balance sheet date of the liability arising from a financial instrument or a contract used to mitigate a specified risk (hedge), and which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.20)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Number 133

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-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-BRD

-Paragraph 41

-IssueDate 2006-05-01

-Chapter 7

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Fair value of liability arising from a financial instrument or contract used to mitigate a specified risk (hedge), expected to be settled after one year or beyond the normal operating cycle, if longer.

+ References

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-Name Accounting Standards Codification

-Topic 825

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-Subparagraph (a)

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