Regulatory Assets Liabilities and Reclassification from Regulatory Assets Liabilities into Cost of Gas Sales (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Natural Gas Future/Options Contract
Regulatory Assets [Line Items]
Loss recognized in Regulatory Assets	$ 0.9		$ (0.3)
Gas Purchase Costs
Regulatory Assets [Line Items]
Loss 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.