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Details on Preferred Stock (Parenthetical) (Detail) (USD $)	3 Months Ended			12 Months Ended		3 Months Ended
	Mar. 31, 2013 Unitil Energy Systems Inc Nonredeemable Preferred Stock Noncumulative Preferred Stock Series 6	Dec. 31, 2012 Unitil Energy Systems Inc Nonredeemable Preferred Stock Noncumulative Preferred Stock Series 6	Mar. 31, 2012 Unitil Energy Systems Inc Nonredeemable Preferred Stock Noncumulative Preferred Stock Series 6	Dec. 31, 2012 Fitchburg Gas and Electric Light Company Series 5	Dec. 31, 2012 Fitchburg Gas and Electric Light Company Series 8	Mar. 31, 2013 Fitchburg Gas and Electric Light Company Redeemable Preferred Stock Cumulative Preferred Stock Series 5	Dec. 31, 2012 Fitchburg Gas and Electric Light Company Redeemable Preferred Stock Cumulative Preferred Stock Series 5	Mar. 31, 2012 Fitchburg Gas and Electric Light Company Redeemable Preferred Stock Cumulative Preferred Stock Series 5	Mar. 31, 2013 Fitchburg Gas and Electric Light Company Redeemable Preferred Stock Cumulative Preferred Stock Series 8	Dec. 31, 2012 Fitchburg Gas and Electric Light Company Redeemable Preferred Stock Cumulative Preferred Stock Series 8	Mar. 31, 2012 Fitchburg Gas and Electric Light Company Redeemable Preferred Stock Cumulative Preferred Stock Series 8
Preferred Units [Line Items]
Dividend rate	6.00%	6.00%	6.00%	5.125%	8.00%	5.125%	5.125%	5.125%	8.00%	8.00%	8.00%
Preferred Stock Par Value	$ 100	$ 100	$ 100			$ 100	$ 100	$ 100	$ 100	$ 100	$ 100

- Definition

Face amount or stated value per share of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer); generally not indicative of the fair market value per share.

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