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Common Stock And Preferred Stock - Additional Information (Detail) - USD ($) |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 29, 2019 | Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018
Class of Stock [Line Items] |  |  |  |
Common stock, shares outstanding |  | 14,916,044 | 14,860,123 | 14,876,955
Common stock, shares issued |  | 5,939 | 7,812 |
Proceeds from Issuance of Common Stock |  | $ 300,000 | $ 300,000 |
Share based compensation expense |  | $ 1,700,000 | 1,800,000 |
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 |  | $ 1,400,000 | 1,000,000 | $ 1,300,000
Preferred Stock |  | 200,000 | 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] |  |  |  |
Proceeds from Issuance of Common Stock |  | $ 314,700 |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Average |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock price per share |  | $ 52.98 |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock, shares issued |  | 5,939 |  |
Unitil Energy Systems Inc | Series 6 |  |  |  |
Class of Stock [Line Items] |  |  |  |
Preferred stock, outstanding |  | 1,893 | 1,893 | 1,893
Preferred Stock |  | $ 200,000 | $ 200,000 | $ 200,000
Dividend rate |  | 6.00% | 6.00% | 6.00%
Restricted Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Restricted stock vesting period |  | 4 years |  |
Restricted stock non-vested |  | 60,496 | 90,882 |
Restricted stock weighted average grant date fair value |  | $ 46.23 | $ 41.93 |
Unrecognized share based compensation |  | $ 1,300,000 |  |
Share compensation recognition period |  | 3 years |  |
Restricted Stock Units Granted | 33,150 |  |  |
Aggregate Market Value | $ 1,600,000 |  |  |
Forfeitures under the stock plan |  | 0 |  |
Cancellations under the stock plan |  | 0 |  |
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% |  |
Restricted Stock Units (RSUs) |  |  |  |
Class of Stock [Line Items] |  |  |  |
Restricted Stock Units Granted |  | 0 |  |
Restricted stock units outstanding |  |  | 52,677 |
Weighted-Average Stock Price |  |  | $ 36.27 |

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