XML 55 R41.htm IDEA: XBRL DOCUMENT v3.5.0.2
Common Stock and Preferred Stock - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Oct. 03, 2016
Apr. 19, 2016
Jan. 26, 2016
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Class of Stock [Line Items]                
Common stock, shares outstanding       14,060,147 13,982,941 14,060,147 13,982,941 13,991,430
Common stock, shares issued           24,697 27,776  
Proceeds from Issuance of Common Stock           $ 1,000,000 $ 1,000,000  
Share based compensation expense           $ 2,000,000 1,600,000  
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled           70.00%    
Percentage of fully-vested restricted stock units that directors will receive in cash when settled           30.00%    
Fair value of liabilities associated with fully vested RSUs that will be settled in cash       $ 600,000 $ 400,000 $ 600,000 400,000 $ 500,000
Preferred Stock       200,000 200,000 200,000 200,000 $ 200,000
Maximum                
Class of Stock [Line Items]                
Dividend declared       $ 100,000 $ 100,000 $ 100,000 $ 100,000  
Series 6 | Unitil Energy Systems Inc                
Class of Stock [Line Items]                
Preferred stock, outstanding       1,893 2,009 1,893 2,009 1,898
Preferred Stock       $ 200,000 $ 200,000 $ 200,000 $ 200,000 $ 200,000
Dividend rate           6.00% 6.00% 6.00%
Dividend and Distribution Reinvestment and Share Purchase Plan                
Class of Stock [Line Items]                
Proceeds from Issuance of Common Stock           $ 985,000    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average                
Class of Stock [Line Items]                
Common stock price per share       $ 39.88   $ 39.88    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock                
Class of Stock [Line Items]                
Common stock, shares issued           24,697    
Restricted Stock                
Class of Stock [Line Items]                
Restricted stock vesting period           4 years    
Restricted stock non-vested       95,506 80,588 95,506 80,588  
Restricted stock weighted average grant date fair value           $ 35.30 $ 33.14  
Unrecognized share based compensation       $ 1,400,000   $ 1,400,000    
Share compensation recognition period           2 years 7 months 6 days    
Forfeitures or cancellations under the stock plan           0    
Fully-vested restricted stock units issued   800 43,220          
Aggregate Market Value     $ 1,600,000          
Restricted Stock | Maximum                
Class of Stock [Line Items]                
Restricted stock available for awards       677,500   677,500    
Restricted stock that may be awarded in any one calendar year to any one participant       20,000   20,000    
Aggregate Market Value   $ 100,000            
Restricted Stock | Vesting Annually                
Class of Stock [Line Items]                
Restricted stock vesting percentage annually           25.00%    
Restricted Stock Units (RSUs)                
Class of Stock [Line Items]                
Fully-vested restricted stock units issued           0    
Restricted stock units outstanding         24,294   24,294  
Weighted-Average Stock Price         $ 30.04   $ 30.04  
Restricted Stock Units (RSUs) | Subsequent Event                
Class of Stock [Line Items]                
Fully-vested restricted stock units issued 12,150