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Common Stock And Preferred Stock - Additional Information (Detail) - USD ($) |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 24, 2023 | Mar. 31, 2023 | Mar. 31, 2022 | Dec. 31, 2022
Class of Stock [Line Items] |  |  |  |
Common stock, shares outstanding |  | 16,086,230 | 16,020,047 | 16,043,355
Common stock, shares issued |  | 5,335 | 5,511 |
Proceeds from Issuance of Common Stock |  | $ 300,000 | $ 300,000 |
Restricted Stock Units Granted |  | $ 48.02 | $ 47.44 |
Share based compensation expense |  | $ 1,100,000 | $ 1,700,000 |
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% |  |
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 |  | 64,243 | 73,178 |
Fair value of liabilities associated with fully vested RSUs that will be settled in cash |  | $ 1,100,000 | $ 1,100,000 | $ 1,000,000.0
Unrecognized share based compensation |  | $ 1,100,000 |  |
Share compensation recognition period |  | 2 years 9 months 18 days |  |
Restricted Stock Units Granted | 18,770 |  |  |
Aggregate Market Value | $ 1,000,000.0 |  |  |
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 |  |  | 49,559 |
Weighted-Average Stock Price |  |  | $ 41.74 |
Performance Shares [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Restricted stock non-vested |  | 18,770 |  |
Restricted Stock Units Granted |  | $ 51.83 |  |
Share based compensation expense |  | $ 100,000 |  |
Unrecognized share based compensation |  | $ 1,200,000 |  |
Share compensation recognition period |  | 2 years 9 months 18 days |  |
Restricted Stock Units Granted | 18,770 |  |  |
Aggregate Market Value | $ 1,000,000.0 |  |  |
Forfeitures under the stock plan |  | 0 |  |
Cancellations under the stock plan |  | 0 |  |
Series 6 | Unitil Energy Systems Inc |  |  |  |
Class of Stock [Line Items] |  |  |  |
Preferred stock, outstanding |  | 1,861 | 1,861 | 1,861
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 |  | $ 289,700 |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Average |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock price per share |  | $ 54.31 |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock, shares issued |  | 5,335 |  |

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