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Regulatory Assets Liabilities and Reclassification from Regulatory Assets Liabilities into Cost of Gas Sales (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Natural Gas Options Contract        
Regulatory Assets [Line Items]        
Loss/(Gain) recognized in Regulatory Assets(Liabilities) $ 0.2 $ 0.1 $ 0.2 $ (0.4)
Gas Purchase Costs        
Regulatory Assets [Line Items]        
Loss/(Gain) Reclassified into unaudited Consolidated Statement of Earnings [1] $ 0.2 $ 0.1 $ 0.2 $ (0.8)
[1] These amounts are offset in the unaudited Consolidated Statements of Earnings with Accrued Revenue and therefore there is no effect on earnings.