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Regulatory Assets Liabilities and Reclassification from Regulatory Assets Liabilities into Cost of Gas Sales (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Natural Gas Futures Contract |  |  |
Regulatory Liabilities [Line Items] |  |  |
Loss/(Gain) recognized in Regulatory Assets(Liabilities) |  | $ 0.4 | $ (0.1)
Gas Purchase Costs |  |  |
Regulatory Liabilities [Line Items] |  |  |
Loss/(Gain) Reclassified into unaudited Consolidated Statement of Earnings | [1] |  | $ 0.3
[1] | These amounts are offset in the 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 gain (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=84234895&loc=SL5624171-113959 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Gains and Losses from hedging program recognized in regulatory assets. + References No definition available. + Details
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