Regulatory Assets Liabilities and Reclassification from Regulatory Assets Liabilities into Cost of Gas Sales (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Natural Gas Futures Contract
Regulatory Liabilities [Line Items]
Loss/(Gain) recognized in Regulatory Assets(Liabilities)		$ 0.4	$ (0.1)
Gas Purchase Costs
Regulatory Liabilities [Line Items]
Loss/(Gain) Reclassified into unaudited Consolidated Statement of Earnings	[1]		$ 0.3

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