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Details on Long Term Debt (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Debt Instrument [Line Items]    
Long-Term Debt $ 287.8 $ 288.3
Less: Current Portion 0.5 0.5
Total Long-term Debt, Less Current Portion 287.3 287.8
Senior Notes Six Point Three Three Percent Due May One Twenty Twenty Two
    
Debt Instrument [Line Items]    
Senior Notes 20.0 20.0
Unitil Energy Systems Inc | First Mortgage Bonds Five Point Two Four Percent Due March Two Twenty Twenty
    
Debt Instrument [Line Items]    
Long-Term Debt 15.0 15.0
Unitil Energy Systems Inc | First Mortgage Bonds Eight Point Four Nine Percent Due October Fourteen Twenty Twenty Four
    
Debt Instrument [Line Items]    
Long-Term Debt 15.0 15.0
Unitil Energy Systems Inc | First Mortgage Bonds Six Point Nine Six Percent Due September One Twenty Twenty Eight
    
Debt Instrument [Line Items]    
Long-Term Debt 20.0 20.0
Unitil Energy Systems Inc | First Mortgage Bonds Eight Point Zero Zero Percent Due May One Twenty Thirty One
    
Debt Instrument [Line Items]    
Long-Term Debt 15.0 15.0
Unitil Energy Systems Inc | First Mortgage Bonds Six Point Three Two Percent Due September Fifteen Twenty Thirty Six
    
Debt Instrument [Line Items]    
Long-Term Debt 15.0 15.0
Fitchburg Gas and Electric Light Company | Notes Six Point Seven Five Percent Due November Thirty Twenty Twenty Three
    
Debt Instrument [Line Items]    
Long-Term Debt 19.0 19.0
Fitchburg Gas and Electric Light Company | Notes Seven Point Three Seven Percent Due January Fifteen Twenty Twenty Nine
    
Debt Instrument [Line Items]    
Long-Term Debt 12.0 12.0
Fitchburg Gas and Electric Light Company | Notes Seven Point Nine Eight Percent Due June One Twenty Thirty One
    
Debt Instrument [Line Items]    
Long-Term Debt 14.0 14.0
Fitchburg Gas and Electric Light Company | Notes Six Point Seven Nine Percent Due October Fifteen Twenty Twenty Five
    
Debt Instrument [Line Items]    
Long-Term Debt 10.0 10.0
Fitchburg Gas and Electric Light Company | Notes Five Point Nine Zero Percent Due December Fifteen Twenty Thirty
    
Debt Instrument [Line Items]    
Long-Term Debt 15.0 15.0
Northern Utilities Inc | Senior Notes Six Point Nine Five Percent Due December Three Twenty Eighteen
    
Debt Instrument [Line Items]    
Senior Notes 30.0 30.0
Northern Utilities Inc | Senior Notes Five Point Two Nine Percent Due March Two Twenty Twenty
    
Debt Instrument [Line Items]    
Senior Notes 25.0 25.0
Northern Utilities Inc | Senior Notes Seven Point Seven Two Percent Due December Three Twenty Thirty Eight
    
Debt Instrument [Line Items]    
Senior Notes 50.0 50.0
Granite State Gas Transmission Inc | Senior Notes Seven Point One Five Percent Due December Fifteen Twenty Eighteen
    
Debt Instrument [Line Items]    
Senior Notes 10.0 10.0
Unitil Realty Corp | Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen
    
Debt Instrument [Line Items]    
Senior Secured Notes $ 2.8 $ 3.3