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Derivative instruments (Details 2) (USD $) In Thousands, unless otherwise specified | Jun. 30, 2012 | Dec. 31, 2011 | Jun. 30, 2011
--- | --- | --- | ---
Derivatives, Fair Value [Line Items] |  |  |
Derivative asset, fair value, gross asset | $ 48,657 | $ 30,455 | $ 20,499
Derivative liability fair value gross liability | 8,000 | 18,863 | 25,421
Designated as hedging instrument [Member] |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative instruments in hedges, assets, at fair value | 47,805 | 30,455 | 20,305
Derivative instruments in hedges, liabilities, at fair value | 7,752 | 18,426 | 24,515
Designated as hedging instrument [Member] | Commodity derivative instruments - current assets [Member] | Commodity contract [Member] |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative instruments in hedges, assets, at fair value | 36,360 | 27,687 | 14,040
Designated as hedging instrument [Member] | Other assets noncurrent [Member] | Commodity contract [Member] |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative instruments in hedges, assets, at fair value | 11,445 | 2,768 | 6,265
Designated as hedging instrument [Member] | Commodity derivative instruments - current liabilities [Member] | Commodity contract [Member] |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative instruments in hedges, liabilities, at fair value | 789 | 12,727 | 17,780
Designated as hedging instrument [Member] | Other accrued liabilities [Member] | Interest rate swap [Member] |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative instruments in hedges, liabilities, at fair value | 6,963 | 827 | 0
Designated as hedging instrument [Member] | Other liabilities noncurrent [Member] | Commodity contract [Member] |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative instruments in hedges, liabilities, at fair value | 0 | 937 | 6,735
Designated as hedging instrument [Member] | Other liabilities noncurrent [Member] | Interest rate swap [Member] |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative instruments in hedges, liabilities, at fair value | 0 | 3,935 | 0
Not designated as hedging instrument [Member] |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Other derivatives not designated as hedging instruments assets at fair value | 852 | 0 | 194
Other derivatives not designated as hedging instruments liabilities at fair value | 248 | 437 | 906
Not designated as hedging instrument [Member] | Commodity derivative instruments - current assets [Member] | Commodity contract [Member] |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Other derivatives not designated as hedging instruments assets at fair value | 640 | 0 | 194
Not designated as hedging instrument [Member] | Other assets noncurrent [Member] | Commodity contract [Member] |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Other derivatives not designated as hedging instruments assets at fair value | 212 | 0 | 0
Not designated as hedging instrument [Member] | Commodity derivative instruments - current liabilities [Member] | Commodity contract [Member] |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Other derivatives not designated as hedging instruments liabilities at fair value | 248 | 437 | 906
Not designated as hedging instrument [Member] | Other liabilities noncurrent [Member] | Commodity contract [Member] |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Other derivatives not designated as hedging instruments liabilities at fair value | $ 0 | $ 0 | $ 0

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- Definition Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 205G -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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44C -Subparagraph a -Clause 1 -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. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624163-113959 + Details
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- Definition Fair value of derivative liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 205G -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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44C -Subparagraph a -Clause 1 -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. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624163-113959 + Details
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- Definition Aggregate fair value of all derivative assets designated as hedging instruments. Includes instruments designated as cash flow hedges, fair value hedges, and hedges of net investments in foreign operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611 Reference 2: 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. + Details
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- Definition Aggregate fair value of all derivative liabilities designated as hedging instruments. Includes instruments designated as cash flow hedges, fair value hedges, and hedges of net investments in foreign operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611 Reference 2: 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. + Details
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- Definition Fair value of all derivative assets not designated as a hedging instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611 Reference 2: 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. + Details
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- Definition Fair value of all derivative liabilities not designated as a hedging instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611 Reference 2: 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. + Details
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