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Derivative instruments (Details) (USD $) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2013 | Jun. 30, 2012 MMBTU | Jun. 30, 2013 | Jun. 30, 2012 MMBTU | Dec. 31, 2012
Derivative instruments [Line Items] |  |  |  |  |
Natural gas swap agreement total forward notional volumes in MMBtu |  | 123,000 |  | 123,000 |
Change in fair value of derivative instruments not designated as hedging instruments recorded as increase (decrease) to regulatory assets |  | $ (261,000) |  | $ (209,000) |
Derivative instruments with credit-risk contingent features, net liability position, aggregate fair value |  | 228,000 |  | 228,000 |
Aggregate fair value of assets needed for immediate settlement if credit-risk-related contingent features were triggered |  | 228,000 |  | 228,000 |
Unrealized gains (losses) included in accumulated other comprehensive income (loss) to be transferred to earnings |  |  | 1,800,000 |  |
Amount of time remaining to transfer accumulated other comprehensive income to earnings |  |  | 18 months |  |
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings | 1,500,000 |  |  |  |
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value | 1,400,000 | 7,800,000 | 1,400,000 | 7,800,000 | 6,300,000
Assets needed for immediate settlement, of credit-risk-related contingent features, aggregate fair value | 1,400,000 | 7,800,000 | 1,400,000 | 7,800,000 | 6,300,000
Oil swap and\or collar [Member] |  |  |  |  |
Derivative instruments [Line Items] |  |  |  |  |
Derivative, nonmonetary notional amount | 3,100,000 | 3,700,000 | 3,100,000 | 3,700,000 | 2,600,000
Natural gas swap and\or collar [Member] |  |  |  |  |
Derivative instruments [Line Items] |  |  |  |  |
Derivative, nonmonetary notional amount | 21,400,000 | 9,100,000 | 21,400,000 | 9,100,000 | 11,000,000
Natural gas basis swap [Member] |  |  |  |  |
Derivative instruments [Line Items] |  |  |  |  |
Derivative, nonmonetary notional amount |  | 1,700,000 |  | 1,700,000 |
Interest rate swap [Member] |  |  |  |  |
Derivative instruments [Line Items] |  |  |  |  |
Derivative, notional amount |  | $ 60,000,000 |  | $ 60,000,000 | $ 50,000,000

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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 -Section 13 -Sentence Column B + Details
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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + 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 No definition available. + Details
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- Definition Change in fair value of derivative instruments not designated as hedging instruments recorded as an increase (decrease) to regulatory assets. + 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 Natural gas swap agreement total forward notional volumes in MMBtu. + 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=7476318&loc=SL5624258-113959 + Details
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- Definition The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7668309&loc=d3e80748-113994 + Details
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- Definition The estimated value of gains (losses), net anticipated to be transferred in the future from accumulated other comprehensive income into earnings. + 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 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=7476318&loc=SL5624258-113959 + Details
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