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Equity - Additional Information (Detail) (USD $)
 12 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Apr. 21, 2011
Dec. 31, 2010
Stock Option
Dec. 31, 2012
Non Vested Restricted Stock
Dec. 31, 2011
Non Vested Restricted Stock
Dec. 31, 2010
Non Vested Restricted Stock
Dec. 31, 2012
Unitil Energy Systems Inc
Series 6
Nonredeemable Preferred Stock
Noncumulative Preferred Stock
Dec. 31, 2012
Restricted Stock Unit
Maximum
Apr. 21, 2011
Before Amendment
May 16, 2012
Common Stock Offering
Oct. 31, 2012
Restricted Stock
Dec. 31, 2012
Restricted Stock
Dec. 31, 2011
Restricted Stock
Dec. 31, 2010
Restricted Stock
Oct. 31, 2012
Restricted Stock
Common Stock
Oct. 31, 2012
Restricted Stock
Cash
Dec. 31, 2012
Restricted Stock
Maximum
Dec. 31, 2011
Restricted Stock Units Issued
Dec. 31, 2010
Restricted Stock Units Issued
Dec. 31, 2012
Series 8
Fitchburg Gas and Electric Light Company
Dec. 31, 2012
Series 5
Fitchburg Gas and Electric Light Company
Dec. 31, 2012
Dividend and Distribution Reinvestment and Share Purchase Plan
Dec. 31, 2011
Dividend and Distribution Reinvestment and Share Purchase Plan
Dec. 31, 2010
Dividend and Distribution Reinvestment and Share Purchase Plan
Class of Stock [Line Items]                                                    
Common stock, authorized shares       25,000,000             16,000,000                              
Common stock, shares outstanding 13,780,601 10,954,065                                                
Common stock, share issued                       2,760,000                       41,752 39,473 41,455
Common stock price per share                       $ 25.25                       $ 26.37    
Proceed from issuance of common stock                       $ 65,700,000                       $ 1,100,000 $ 1,000,000 $ 900,000
Common stock shares repurchase 6,368 8,765 3,225                                              
Repurchase expense 200,000 200,000 100,000                                              
Restricted stock available for Awards                                     677,500              
Restricted stock that may be awarded in any one calendar year to any one participant                                     20,000              
Restricted stock Vesting Period                           4 years                        
Restricted stock Vesting Percentage Annually                           25.00%                        
Share Based compensation Expense                           1,300,000 700,000 500,000                    
Restricted Stock Non-Vested                           53,932 52,362                      
Restricted Stock Weighted Average Grant Date Fair Value                           $ 24.67 $ 22.21                      
Total unrecognized compensation cost                           700,000                        
Share Compensation Recognition period                           2 years 6 months                        
Restricted shares forfeited                           816                        
Restricted shares cancellation                           0                        
Restricted stock Units Issued                                       0 0          
Fully-vested restricted stock units issued                         5,470                          
Percentage underlying restricted stock that directors will receive                                 70.00% 30.00%                
Fair value of liabilities associated with fully vested RSUs that will be settled in cash                   100,000                                
Preferred Stock 200,000 2,000,000             200,000                                  
Dividend rate                 6.00%                         8.00% 5.125%      
Redeemable, Cumulative Preferred Stock $ 8,000 $ 8,600 $ 25,000                                     $ 965,400 $ 792,313      
Preferred Stock premium percentage                                             1.28%      
Antidilutive securities excluded from computation of earnings per share         33,000 24,325 1,642 6,164