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Common Stock And Preferred Stock - Additional Information (Detail) - USD ($) |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 26, 2021 | Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Class of Stock [Line Items] |  |  |  |
Common stock, shares outstanding |  | 15,041,629 | 14,963,444 | 15,012,310
Common stock, shares issued |  | 6,179 | 4,644 |
Proceeds from Issuance of Common Stock |  | $ 300,000 | $ 300,000 |
Restricted Stock Units Granted |  | $ 49.44 | $ 54.88 |
Share based compensation expense |  | $ 900,000 | $ 1,800,000 |
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled |  | 70.00% |  |
Fair value of liabilities associated with fully vested RSUs that will be settled in cash |  | $ 900,000 | 1,600,000 | $ 800,000
Percentage of fully-vested restricted stock units that directors will receive in cash when settled |  | 30.00% |  |
Preferred Stock |  | $ 200,000 | 200,000 | $ 200,000
Maximum |  |  |  |
Class of Stock [Line Items] |  |  |  |
Dividend declared |  | $ 100,000 | $ 100,000 |
Restricted Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Restricted stock vesting period |  | 4 years |  |
Restricted stock non-vested |  | 62,566 | 56,813 |
Unrecognized share based compensation |  | $ 1,200,000 |  |
Share compensation recognition period |  | 3 years |  |
Restricted Stock Units Granted | 23,140 |  |  |
Aggregate Market Value | $ 900,000 |  |  |
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 |  |  | 70,833 |
Weighted-Average Stock Price |  |  | $ 41.30 |
Series 6 | Unitil Energy Systems Inc |  |  |  |
Class of Stock [Line Items] |  |  |  |
Preferred stock, outstanding |  | 1,887 | 1,887 | 1,887
Preferred Stock |  | $ 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 |  | $ 266,800 |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Average |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock price per share |  | $ 43.17 |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock, shares issued |  | 6,179 |  |

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