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Regulatory Assets (Liabilities) and Reclassification from Regulatory Asset (Liabilities) into Purchased Gas (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012
Natural Gas Future Contract |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Loss (Gain) recognized in Regulatory Assets (Liabilities) | $ (0.3) |  | $ 1.3 |
Gas Purchase Costs |  |  |  |
Regulatory Assets [Line Items] |  |  |  |
Loss Reclassified into unaudited Consolidated Statement of Earnings | $ 0.9 | [1] | $ 1.6 | [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 Loss included in earnings for the period from the change in fair value of derivative instruments not designated 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 45 -Subparagraph b(1)(b) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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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