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Details on Long Term Debt (Parenthetical) (Detail)
Dec. 31, 2012
Dec. 31, 2011
Senior Notes Six Point Three Three Percent Due May One Twenty Twenty Two
    
Debt Instrument [Line Items]    
Stated interest rate 6.33% 6.33%
Unitil Energy Systems Inc | First Mortgage Bonds Five Point Two Four Percent Due March Two Twenty Twenty
    
Debt Instrument [Line Items]    
Stated interest rate 5.24% 5.24%
Unitil Energy Systems Inc | First Mortgage Bonds Eight Point Four Nine Percent Due October Fourteen Twenty Twenty Four
    
Debt Instrument [Line Items]    
Stated interest rate 8.49% 8.49%
Unitil Energy Systems Inc | First Mortgage Bonds Six Point Nine Six Percent Due September One Twenty Twenty Eight
    
Debt Instrument [Line Items]    
Stated interest rate 6.96% 6.96%
Unitil Energy Systems Inc | First Mortgage Bonds Eight Point Zero Zero Percent Due May One Twenty Thirty One
    
Debt Instrument [Line Items]    
Stated interest rate 8.00% 8.00%
Unitil Energy Systems Inc | First Mortgage Bonds Six Point Three Two Percent Due September Fifteen Twenty Thirty Six
    
Debt Instrument [Line Items]    
Stated interest rate 6.32% 6.32%
Fitchburg Gas and Electric Light Company | Notes Six Point Seven Five Percent Due November Thirty Twenty Twenty Three
    
Debt Instrument [Line Items]    
Stated interest rate 6.75% 6.75%
Fitchburg Gas and Electric Light Company | Notes Seven Point Three Seven Percent Due January Fifteen Twenty Twenty Nine
    
Debt Instrument [Line Items]    
Stated interest rate 7.37% 7.37%
Fitchburg Gas and Electric Light Company | Notes Seven Point Nine Eight Percent Due June One Twenty Thirty One
    
Debt Instrument [Line Items]    
Stated interest rate 7.98% 7.98%
Fitchburg Gas and Electric Light Company | Notes Six Point Seven Nine Percent Due October Fifteen Twenty Twenty Five
    
Debt Instrument [Line Items]    
Stated interest rate 6.79% 6.79%
Fitchburg Gas and Electric Light Company | Notes Five Point Nine Zero Percent Due December Fifteen Twenty Thirty
    
Debt Instrument [Line Items]    
Stated interest rate 5.90% 5.90%
Northern Utilities Inc | Senior Notes Six Point Nine Five Percent Due December Three Twenty Eighteen
    
Debt Instrument [Line Items]    
Stated interest rate 6.95% 6.95%
Northern Utilities Inc | Senior Notes Five Point Two Nine Percent Due March Two Twenty Twenty
    
Debt Instrument [Line Items]    
Stated interest rate 5.29% 5.29%
Northern Utilities Inc | Senior Notes Seven Point Seven Two Percent Due December Three Twenty Thirty Eight
    
Debt Instrument [Line Items]    
Stated interest rate 7.72% 7.72%
Granite State Gas Transmission Inc | Senior Notes Seven Point One Five Percent Due December Fifteen Twenty Eighteen
    
Debt Instrument [Line Items]    
Stated interest rate 7.15% 7.15%
Unitil Realty Corp | Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen
    
Debt Instrument [Line Items]    
Stated interest rate 8.00% 8.00%