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Bad Debts - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Financing Receivable, Impaired [Line Items]
Provision for Bad Debt	$ 3.6	$ 4.3	$ 4.5
Unitil Energy Systems Inc
Financing Receivable, Impaired [Line Items]
Provision for Bad Debt	$ 1.9	$ 1.8	$ 1.4

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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