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Derivative instruments (Details) (USD $) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2011 | Sep. 30, 2010 | Sep. 30, 2011 | Sep. 30, 2010 | Dec. 31, 2010
Derivative instruments [Abstract] |  |  |  |  |
Forward notional volumes of natural gas swap agreements for derivative instruments not designated as hedging instruments in MMBtu's | 676,000 |  | 676,000 |  |
Change in fair value of derivative instruments not designated as hedging instruments recorded as an increase (decrease) to regulatory assets | $ 414,000 | $ 2,700,000 | $ (8,100,000) | $ (6,300,000) |
Derivative instruments with credit-risk-related contingent features, net liability position, aggregate fair value | 1,300,000 |  | 1,300,000 |  |
Aggregate Fair Value for Cascade and/or Intermountain of Assets Needed for Immediate Settlement if credit-risk-related contingent features were triggered | 1,300,000 |  | 1,300,000 |  |
Nonmonetary notional forward amount of natural gas swap, collar and/or put agreements in MMBtu, derivative instruments designated as cash flow hedging instruments | 16,400,000 |  | 16,400,000 |  |
Nonmonetary Notional forward Amount of natural gas basis swap agreements in million MMBtu, Derivative Instruments Designated as cash flow hedging Instruments | 8,200,000 |  | 8,200,000 |  |
Nonmonetary Notional forward Amount of oil swap, collar and/or put agreements in Bbl, Derivative Instruments Designated as cash flow hedging Instruments | 3,100,000 |  | 3,100,000 |  |
Oil call option agreement with total forward notional volumes (in Bbl) | 92,000 |  | 92,000 |  |
Notional amount of interest rate swap agreements | 60,000,000 |  | 60,000,000 |  |
Gain on the derivative instrument that did not qualify for hedge accounting reported in operating revenues | 157,000 |  | 336,000 |  |
Maximum Length of Time Hedged in swap, collar and/or put cash flow hedge agreements in months |  |  | 27 |  |
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months from AOCI into earnings |  |  | 23,000,000 |  |
Derivative instruments with credit-risk-related contingent features, Net Liability Position, Aggregate Fair Value for Fidelity and Centennial | 5,200,000 |  | 5,200,000 |  |
Assets Needed for Immediate Settlement if credit-risk-related contingent feature triggered, Aggregate Fair Value for Fidelity and Centennial | 5,200,000 |  | 5,200,000 |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Asset Derivatives Designated as Hedges | 54,033,000 | 35,226,000 | 54,033,000 | 35,226,000 | 19,227,000
Asset Derivatives Not Designated as Hedges | 336,000 | 0 | 336,000 | 0 | 0
Total Asset Derivatives | 54,369,000 | 35,226,000 | 54,369,000 | 35,226,000 | 19,227,000
Liability Derivatives Designated as Hedges | 5,371,000 | 6,494,000 | 5,371,000 | 6,494,000 | 21,552,000
Liability Derivatives Not Designated as Hedges | 1,305,000 | 21,572,000 | 1,305,000 | 21,572,000 | 9,359,000
Total Liability Derivatives | 6,676,000 | 28,066,000 | 6,676,000 | 28,066,000 | 30,911,000
Commodity Contract [Member] | Other liabilities noncurrent [Member] |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Liability Derivatives Designated as Hedges | 157,000 | 1,845,000 | 157,000 | 1,845,000 | 6,483,000
Liability Derivatives Not Designated as Hedges | 0 | 418,000 | 0 | 418,000 | 0
Commodity Contract [Member] | Other assets noncurrent [Member] |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Asset Derivatives Designated as Hedges | 15,575,000 | 8,423,000 | 15,575,000 | 8,423,000 | 4,104,000
Interest Rate Swap [Member] | Other liabilities noncurrent [Member] |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Liability Derivatives Designated as Hedges | 3,491,000 | 0 | 3,491,000 | 0 | 0
Commodity derivative instruments - current assets [Member] |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Asset Derivatives Designated as Hedges | 38,458,000 | 26,803,000 | 38,458,000 | 26,803,000 | 15,123,000
Asset Derivatives Not Designated as Hedges | 336,000 | 0 | 336,000 | 0 | 0
Commodity derivative instruments - current liabilities [Member] |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Liability Derivatives Designated as Hedges | 1,723,000 | 4,649,000 | 1,723,000 | 4,649,000 | 15,069,000
Liability Derivatives Not Designated as Hedges | $ 1,305,000 | $ 21,154,000 | $ 1,305,000 | $ 21,154,000 | $ 9,359,000

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- Definition Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 + Details
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- Definition Forward notional volumes of natural gas swap agreements for derivative instruments not designated as hedging instruments in MMBtu's + Details
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- Definition The aggregate fair value of assets needed to immediately settle the derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624258-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44D -Subparagraph c -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 The amount of gain recognized in income during the period on derivative instruments designated and qualifying as hedging instruments in fair value hedges and related hedged items designated and qualifying in fair value hedges, on derivative instruments designated and qualifying as hedging instruments in cash flow hedges, and on derivative instruments not designated as hedging instruments. + 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 Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5618551-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624171-113959 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44C -Subparagraph b -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 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 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. Reference 2: 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=6947722&loc=d3e13433-108611 + 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 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. Reference 2: 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=6947722&loc=d3e13433-108611 + Details
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- Definition The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624258-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44D -Subparagraph b -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 Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 45 -Subparagraph b(3) -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 Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6887037&loc=d3e80748-113994 + Details
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- Definition Aggregate notional amount of all derivatives. The notional amount relates to a number of currency units, shares, bushels, pounds, or other units specified in a derivative instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Notional Amount -URI http://asc.fasb.org/extlink&oid=6519104 + 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 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. Reference 2: 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=6947722&loc=d3e13433-108611 + 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 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. Reference 2: 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=6947722&loc=d3e13433-108611 + Details
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