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Derivative instruments (Details) (USD $) | 3 Months Ended
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Mar. 31, 2012 MMBTU | Mar. 31, 2011
Derivative instruments [Line Items] |  |
Natural gas swap agreement total forward notional volumes in MMBtu | 214,000 |
Change in fair value of derivative instruments not designated as hedging instruments recorded as increase (decrease) to regulatory assets | $ 52,000 | $ (6,600,000)
Derivative instruments with credit-risk-related contingent features, net liability position, aggregate fair value | 489,000 |
Aggregate fair value of assets needed for immediate settlement if credit-risk-related contingent features were triggered | 489,000 |
Maximum length of time hedged in swap, collar and/or put cash flow hedge agreements (in months) | 21 |
Notional amount of interest rate derivatives | 60,000,000 |
Amount of loss due to hedge ineffectiveness | 4,300,000 |
Loss on derivatives that did not qualify for hedge accounting |  | 1,700,000
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings | 5,300,000 |
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value | 29,200,000 |
Assets needed for immediate settlement, of credit-risk-related contingent features, aggregate fair value | $ 29,200,000 |
Oil swap and\or collar [Member] |  |
Derivative instruments [Line Items] |  |
Nonmonetary notional amount - derivative instruments | 4,000,000 |
Natural gas swap and\or collar [Member] |  |
Derivative instruments [Line Items] |  |
Nonmonetary notional amount of derivative instruments | 10,900,000 |
Natural gas basis swap [Member] |  |
Derivative instruments [Line Items] |  |
Nonmonetary notional amount of derivative instruments | 2,600,000 |

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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 No definition available. + 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 cash flow hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain or loss on that instrument. + References No definition available. + 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 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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