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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 |  |  |  |  |  |  |

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