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Regulatory Assets and Reclassification From the Regulatory Asset into Purchased Gas (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Natural Gas Future Contract
        
Regulatory Assets [Line Items]        
Gain (loss) recognized in Regulatory Assets $ 0.1 $ 0.5 $ 1.4 $ 0.6
Gas Purchase Costs
        
Regulatory Assets [Line Items]        
Loss reclassification into unaudited Consolidated Statement of Earnings $ 0.6 [1] $ 0.3 [1] $ 2.2 [1] $ 1.0 [1]
[1] These amounts are offset in the unaudited Consolidated Statements of Earnings with Accrued Revenue and therefore there is no effect on earnings.