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Common Stock and Preferred Stock - Additional Information (Detail) (USD $)
 9 Months Ended 3 Months Ended 9 Months Ended 0 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Dec. 31, 2013
Sep. 30, 2014
Maximum
Sep. 30, 2013
Maximum
Sep. 30, 2014
Maximum
Sep. 30, 2013
Maximum
Sep. 30, 2014
Dividend and Distribution Reinvestment and Share Purchase Plan
Sep. 30, 2014
Dividend and Distribution Reinvestment and Share Purchase Plan
Common Stock
Sep. 30, 2014
Restricted Stock
Sep. 30, 2013
Restricted Stock
Sep. 30, 2014
Restricted Stock
Maximum
Sep. 30, 2013
Restricted Stock Units (RSUs)
Oct. 01, 2014
Restricted Stock Units (RSUs)
Subsequent Event
Sep. 30, 2014
Unitil Energy Systems Inc
Series 6
Sep. 30, 2013
Unitil Energy Systems Inc
Series 6
Dec. 31, 2013
Unitil Energy Systems Inc
Series 6
Class of Stock [Line Items]                                  
Common stock, shares outstanding 13,906,964 13,831,624 13,841,400                            
Common stock, share issued 28,958 29,783             28,958                
Common stock price per share               $ 32.20                  
Proceed from issuance of common stock $ 900,000 $ 900,000           $ 932,000                  
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%              
Restricted stock issue date                   Jan. 31, 2014              
Restricted stock issued 35,500                                
Restricted stock grant date market value 1,100,000                                
Restricted stock non-vested                   67,334 53,480            
Restricted stock weighted average grant date fair value                   $ 28.51 $ 25.99            
Share based compensation expense 1,300,000 600,000                              
Unrecognized share based compensation                   900,000              
Share compensation recognition period                   2 years 7 months 6 days              
Forfeitures or cancellations under the stock plan                   0              
Fair value of liabilities associated with cash portion of fully vested RSUs 200,000 100,000 200,000                            
Weighted-Average Stock Price                         $ 27.44        
Restricted stock units outstanding                         4,024        
Fully-vested restricted stock units issued                            12,969      
Preferred stock, outstanding                             2,250 2,250 2,250
Preferred Stock 200,000 200,000 200,000                       200,000 200,000 200,000
Dividend rate                             6.00% 6.00% 6.00%
Dividend declared       $ 100,000 $ 100,000 $ 100,000 $ 100,000