Details on Preferred Stock (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Preferred Units [Line Items]
Preferred Stock	$ 0.2	$ 0.2	$ 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
Fitchburg Gas and Electric Light Company | Redeemable Preferred Stock | Cumulative Preferred Stock | Series 5
Preferred Units [Line Items]
Preferred Stock			0.8
Fitchburg Gas and Electric Light Company | Redeemable Preferred Stock | Cumulative Preferred Stock | Series 8
Preferred Units [Line Items]
Preferred Stock			$ 1.0