Details on Long Term Debt (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Debt Instrument [Line Items]
Short-Term Debt Reclassified to Long-Term Debt	$ 42.3
Long-Term Debt	329.2	287.3	287.5
Less: Current Portion	2.5	2.5	0.6
Total Long-Term Debt, Less Current Portion	326.7	284.8	286.9
Long-Term Debt	329.2	287.3	287.5
Before Reclassification of Short-Term Debt to Long-Term Debt
Debt Instrument [Line Items]
Long-Term Debt	286.9	287.3	287.5
Long-Term Debt	286.9	287.3	287.5
6.33% Notes, Due May 1, 2022
Debt Instrument [Line Items]
Senior Notes	20.0	20.0	20.0
Unitil Energy Systems Inc | First Mortgage Bonds 5.24% Series, Due March 2, 2020
Debt Instrument [Line Items]
Long-Term Debt	15.0	15.0	15.0
Long-Term Debt	15.0	15.0	15.0
Unitil Energy Systems Inc | First Mortgage Bonds 8.49% Series, Due October 14, 2024
Debt Instrument [Line Items]
Long-Term Debt	15.0	15.0	15.0
Long-Term Debt	15.0	15.0	15.0
Unitil Energy Systems Inc | First Mortgage Bonds 6.96% Series, Due September 1, 2028
Debt Instrument [Line Items]
Long-Term Debt	20.0	20.0	20.0
Long-Term Debt	20.0	20.0	20.0
Unitil Energy Systems Inc | First Mortgage Bonds 8.00% Series, Due May 1, 2031
Debt Instrument [Line Items]
Long-Term Debt	15.0	15.0	15.0
Long-Term Debt	15.0	15.0	15.0
Unitil Energy Systems Inc | First Mortgage Bonds 6.32% Series, Due September 15, 2036
Debt Instrument [Line Items]
Long-Term Debt	15.0	15.0	15.0
Long-Term Debt	15.0	15.0	15.0
Fitchburg Gas and Electric Light Company | 6.75% Notes, Due November 30, 2023
Debt Instrument [Line Items]
Long-Term Debt	19.0	19.0	19.0
Long-Term Debt	19.0	19.0	19.0
Fitchburg Gas and Electric Light Company | 7.37% Notes, Due January 15, 2029
Debt Instrument [Line Items]
Long-Term Debt	12.0	12.0	12.0
Long-Term Debt	12.0	12.0	12.0
Fitchburg Gas and Electric Light Company | 7.98% Notes, Due June 1, 2031
Debt Instrument [Line Items]
Long-Term Debt	14.0	14.0	14.0
Long-Term Debt	14.0	14.0	14.0
Fitchburg Gas and Electric Light Company | 6.79% Notes, Due October 15, 2025
Debt Instrument [Line Items]
Long-Term Debt	10.0	10.0	10.0
Long-Term Debt	10.0	10.0	10.0
Fitchburg Gas and Electric Light Company | 5.90% Notes, Due December 15, 2030
Debt Instrument [Line Items]
Long-Term Debt	15.0	15.0	15.0
Long-Term Debt	15.0	15.0	15.0
Northern Utilities Inc | 6.95% Senior Notes, Due December 3, 2018
Debt Instrument [Line Items]
Senior Notes	30.0	30.0	30.0
Northern Utilities Inc | 5.29% Senior Notes, Due March 2, 2020
Debt Instrument [Line Items]
Senior Notes	25.0	25.0	25.0
Northern Utilities Inc | 7.72% Senior Notes, Due December 3, 2038
Debt Instrument [Line Items]
Senior Notes	50.0	50.0	50.0
Granite State Gas Transmission Inc | 7.15% Senior Notes, Due December 15, 2018
Debt Instrument [Line Items]
Senior Notes	10.0	10.0	10.0
Unitil Realty Corp | 8.00% Senior Secured Notes, Due Through August 1, 2017
Debt Instrument [Line Items]
Senior Secured Notes	$ 1.9	$ 2.3	$ 2.5