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Regulatory Assets and Reclassification from Regulatory Asset into Purchased Gas (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Natural Gas Future Contract
    
Regulatory Assets [Line Items]    
Gain (loss) recognized in Regulatory Assets $ 1.0 $ 2.9
Gas Purchase Costs
    
Regulatory Assets [Line Items]    
Loss reclassification into unaudited Consolidated Statement of Earnings $ 2.6 [1] $ 1.6 [1]
[1] These amounts are offset in the Consolidated Statements of Earnings with the recognition of accrued revenue as a component of Gas Operating Revenue and therefore there is no effect on earnings.