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Details on Long Term Debt (Parenthetical) (Detail)
Mar. 31, 2013
Dec. 31, 2012
Mar. 31, 2012
Senior Notes Six Point Three Three Percent Due May One Twenty Twenty Two
      
Debt Instrument [Line Items]      
Stated interest rate 6.33% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 8.00%