Regulatory Assets and Reclassification From the Regulatory Asset into Purchased Gas (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012		Sep. 30, 2011
Natural Gas Future Contract
Regulatory Assets [Line Items]
Gain (loss) recognized in Regulatory Assets	$ (0.3)	$ 0.9	$ 1.1		$ 1.5
Gas Purchase Costs
Regulatory Assets [Line Items]
Loss reclassification into unaudited Consolidated Statement of Earnings			$ 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.