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Details on Long Term Debt (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Mar. 31, 2012
Debt Instrument [Line Items]      
Long-Term Debt $ 287.7 $ 287.8 $ 288.2
Less: Current Portion 0.5 0.5 0.5
Total Long-term Debt, Less Current Portion 287.2 287.3 287.7
Senior Notes Six Point Three Three Percent Due May One Twenty Twenty Two
      
Debt Instrument [Line Items]      
Senior Notes 20.0 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 $ 2.7 $ 2.8 $ 3.2