Details on Preferred Stock (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Preferred Units [Line Items]
Preferred Stock	$ 2.0	$ 2.0	$ 2.0
Unitil Energy Systems Inc | Nonredeemable Preferred Stock | Noncumulative Preferred Stock | Series 6
Preferred Units [Line Items]
Preferred Stock	0.2	0.2	0.2
Preferred Stock	2,250	2,250	2,250
Fitchburg Gas and Electric Light Company | Redeemable Preferred Stock | Cumulative Preferred Stock | Series 5
Preferred Units [Line Items]
Preferred Stock	0.8	0.8	0.8
Preferred Stock	7,823	7,861	7,861
Fitchburg Gas and Electric Light Company | Redeemable Preferred Stock | Cumulative Preferred Stock | Series 8
Preferred Units [Line Items]
Preferred Stock	$ 1.0	$ 1.0	$ 1.0
Preferred Stock	9,654	9,696	9,696