Regulatory Assets Liabilities and Reclassification from Regulatory Assets Liabilities into Cost of Gas Sales (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014		Jun. 30, 2013
Natural Gas Future/Options Contract
Regulatory Assets [Line Items]
Loss/(Gain) recognized in Regulatory Assets	$ 0.4	$ 0.9	$ (0.5)		$ 0.6
Gas Purchase Costs
Regulatory Assets [Line Items]
Loss/(Gain) Reclassified into unaudited Consolidated Statement of Earnings			$ (0.9)	[1]	$ 0.9	[1]

[1] These amounts are offset in the unaudited Consolidated Statements of Earnings with Accrued Revenue and therefore there is no effect on earnings.