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Equity - Additional Information (Detail) - USD ($) |  |  | 12 Months Ended
--- | --- | --- | ---
Jan. 29, 2018 | Dec. 14, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Class of Stock [Line Items] |  |  |  |  |
Common stock, shares outstanding |  |  | 14,815,585 | 14,065,230 |
Common stock, shares authorized |  |  | 25,000,000 | 25,000,000 |
Common stock, shares issued |  |  |  | 32,095 | 36,265
Proceeds from Issuance of Common Stock |  |  | $ 33,000,000 | $ 1,300,000 | $ 1,300,000
Common stock shares repurchase |  |  | 1,686 | 1,949 | 1,981
Repurchase expense |  |  | $ 100,000 | $ 100,000 | $ 100,000
Share based compensation expense |  |  | $ 2,700,000 | 2,200,000 | 1,900,000
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 |  |  | $ 1,000,000 | 800,000 |
Preferred Stock |  |  | 200,000 | 200,000 |
Maximum |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Dividend declared |  |  | $ 100,000 | 100,000 |
Dividend and Distribution Reinvestment and Share Purchase Plan |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Common stock, shares issued |  |  | 26,256 |  |
Proceeds from Issuance of Common Stock |  |  | $ 1,300,000 | $ 1,300,000 | $ 1,300,000
Dividend and Distribution Reinvestment and Share Purchase Plan | Average |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Common stock price per share |  |  | $ 47.71 |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Common stock, shares issued |  |  | 26,256 | 32,095 | 36,265
Registered Public Offering |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Common stock, shares issued |  |  | 690,000 |  |
Proceeds from Issuance of Common Stock |  |  | $ 31,700,000 |  |
Registered Public Offering | Common Stock |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Common stock, shares issued |  | 690,000 |  |  |
Common stock price per share |  | $ 48.30 |  |  |
Unitil Energy Systems Inc | Series 6 |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Preferred stock, outstanding |  |  | 1,893 | 1,893 |
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 |  |  | 89,326 | 93,747 |
Restricted stock weighted average grant date fair value |  |  | $ 39.54 | $ 35.29 |
Unrecognized share based compensation |  |  | $ 800,000 |  |
Share compensation recognition period |  |  | 2 years 3 months 19 days |  |
Forfeitures under the stock plan |  |  | 831 |  |
Cancellations under the stock plan |  |  | 0 |  |
Restricted Stock | Subsequent Event |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Restricted Stock Units Granted | 37,510 |  |  |  |
Aggregate Market Value | $ 1,600,000 |  |  |  |
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% |  |

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