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Equity - Additional Information (Detail) - USD ($) |  | 12 Months Ended
--- | --- | ---
Jan. 26, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Class of Stock [Line Items] |  |  |  |
Common stock, shares outstanding |  | 13,991,430 | 13,916,026 |
Common stock, share authorized |  | 25,000,000 | 25,000,000 |
Common stock, shares issued |  | 36,265 | 38,020 | 39,559
Proceeds from Issuance of Common Stock |  | $ 1,300,000 | $ 1,200,000 | $ 1,100,000
Common stock shares repurchase |  | 1,981 | 2,763 | 2,969
Repurchase expense |  | $ 100,000 | $ 100,000 | $ 100,000
Share based compensation expense |  | $ 1,900,000 | 1,400,000 | $ 700,000
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 |  | $ 500,000 | 400,000 |
Preferred Stock |  | 200,000 | 200,000 |
Maximum |  |  |  |
Class of Stock [Line Items] |  |  |  |
Dividend declared |  | $ 100,000 | $ 100,000 |
Non Vested Restricted Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Antidilutive securities excluded from computation of earnings per share |  | 36,941 | 0 | 4,481
Dividend and Distribution Reinvestment and Share Purchase Plan |  |  |  |
Class of Stock [Line Items] |  |  |  |
Proceeds from Issuance of Common Stock |  | $ 1,300,000 | $ 1,200,000 | $ 1,100,000
Dividend and Distribution Reinvestment and Share Purchase Plan | Average |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock price per share |  | $ 34.77 |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock, shares issued |  | 36,265 | 38,020 | 39,559
Unitil Energy Systems Inc | Series 6 |  |  |  |
Class of Stock [Line Items] |  |  |  |
Preferred stock, outstanding |  | 1,898 | 2,250 |
Preferred Stock |  | $ 200,000 | $ 200,000 |
Dividend rate |  | 6.00% | 6.00% |
Restricted Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Restricted stock vesting period |  | 4 years |  |
Restricted stock non-vested |  | 70,761 | 67,334 |
Restricted stock weighted average grant date fair value |  | $ 32.56 | $ 28.51 |
Unrecognized share based compensation |  | $ 1,100,000 |  |
Share compensation recognition period |  | 2 years 4 months 24 days |  |
Forfeitures or cancellations under the stock plan |  | 871 |  |
Restricted Stock | Maximum |  |  |  |
Class of Stock [Line Items] |  |  |  |
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 Annually |  |  |  |
Class of Stock [Line Items] |  |  |  |
Restricted stock vesting percentage annually |  | 25.00% |  |
Subsequent Event | Restricted Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Shares | 43,220 |  |  |
Aggregate Market Value | $ 1,600,000 |  |  |

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