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Regulatory Assets Liabilities and Reclassification from Regulatory Assets Liabilities into Purchased Gas (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended
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Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 | Sep. 30, 2012
Natural Gas Future Contract |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |
Loss (Gain) recognized in Regulatory Assets | $ 0.1 |  | $ (0.7) | $ 0.7 |  | $ 0.7 |
Gas Purchase Costs |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |
Loss Reclassified into unaudited Consolidated Statement of Earnings | $ 0.2 | [1] |  | $ 1.1 | [1] | $ 2.2 | [1]
[1] | These amounts are offset in the unaudited Consolidated Statements of Earnings with Accrued Revenue and therefore there is no effect on earnings.

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- Definition Amount of realized and unrealized loss of derivative instruments not designated or qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959 + Details
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- Details
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- Definition Derivative Instruments Gain Loss Recognized In Regulatory Assets + References No definition available. + Details
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