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Activity in Company's Allowance for Doubtful Accounts (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Electric	Dec. 31, 2011 Electric	Dec. 31, 2010 Electric	Dec. 31, 2012 Gas	Dec. 31, 2011 Gas	Dec. 31, 2010 Gas	Dec. 31, 2012 Others	Dec. 31, 2011 Others	Dec. 31, 2010 Others	Dec. 31, 2009 Others
Accounts and Other Receivables [Line Items]
Balance at Beginning of Period	$ 2.3	$ 2.6	$ 2.5	$ 1.7	$ 1.8	$ 1.7	$ 0.5	$ 0.7	$ 0.7	$ 0.1	$ 0.1	$ 0.1	$ 0.1
Provision	3.6	4.3	4.5	1.4	2.1	2.0	2.2	2.2	2.5
Recoveries	0.6	0.5	0.6	0.3	0.2	0.2	0.3	0.3	0.4
Accounts Written Off	4.6	5.1	5.0	2.3	2.4	2.1	2.3	2.7	2.9
Balance at End of Period	$ 1.9	$ 2.3	$ 2.6	$ 1.1	$ 1.7	$ 1.8	$ 0.7	$ 0.5	$ 0.7	$ 0.1	$ 0.1	$ 0.1	$ 0.1

Amount of the current period expense charged against operations, the offset which is generally to the allowance for doubtful accounts for the purpose of reducing receivables, including notes receivable, to an amount that approximates their net realizable value (the amount expected to be collected).

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