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Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions |  |  | 12 Months Ended
--- | --- | --- | ---
Jan. 24, 2023 | Aug. 06, 2021 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Class of Stock [Line Items] |  |  |  |  |
Common Stock, Shares, Outstanding |  |  | 16,043,355 | 15,977,766 |
Common stock, shares issued |  |  | 18,853 | 942,316 | 23,658
Common stock, shares authorized |  |  | 25,000,000 | 25,000,000 |
Proceeds from Issuance of Common Stock |  |  | $ 1.0 | $ 45.5 | $ 1.1
Restricted stock weighted average grant date fair value |  |  | $ 46.45 | $ 49.72 |
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled |  |  | 70.00% |  |
Common stock shares repurchase |  |  | 9,449 | 8,012 | 13,194
Share based compensation expense |  |  | $ 2.1 | $ 1.4 | $ 2.2
Percentage of fully-vested restricted stock units that directors will receive in cash when settled |  |  | 30.00% |  |
Preferred Stock |  |  | $ 0.2 | $ 0.2 |
Restricted Stock |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Restricted stock vesting period |  |  | 4 years |  |
Restricted stock non-vested |  |  | 45,473 | 37,621 |
Fair value of liabilities associated with fully vested RSUs that will be settled in cash |  |  | $ 1.0 | $ 1.0 |
Unrecognized share based compensation |  |  | $ 0.8 |  |
Share compensation recognition period |  |  | 2 years 6 months |  |
Cancellations under the stock plan |  |  | 0 |  |
Restricted forfeited shares |  |  | 270 |  |
Restricted Stock | Subsequent Event |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Restricted Stock Units Granted | 18,770 |  |  |  |
Aggregate Market Value | $ 1.0 |  |  |  |
Restricted Stock | Vesting Annually |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Restricted stock vesting percentage annually |  |  | 25.00% |  |
Performance Restricted Shares | Subsequent Event |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Restricted Stock Units Granted | 18,770 |  |  |  |
Aggregate Market Value | $ 1.0 |  |  |  |
Unitil Energy Systems Inc | Series 6 |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Preferred stock, outstanding |  |  | 1,861 | 1,861 |
Preferred Stock |  |  | $ 0.2 | $ 0.2 |
Dividend rate |  |  | 6.00% | 6.00% |
Dividend and Distribution Reinvestment and Share Purchase Plan |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Proceeds from Issuance of Common Stock |  |  | $ 1.0 | $ 1.0 | $ 1.1
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Common stock, shares issued |  |  | 18,853 | 22,316 | 23,658
Over-Allotment Option [Member] |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Number Of Shares Granted Overallotment Option To Purchase Additional Shares |  | 30 days |  |  |
Sale of Stock, Number of Shares Issued in Transaction |  | 120,000 |  |  |
Proceeds from Issuance of Private Placement |  | $ 5.9 |  |  |
Public Offering [Member] | Common Stock |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Common stock, shares issued |  | 800,000 |  |  |
Common stock price per share |  | $ 50.80 |  |  |
Proceeds from Issuance Initial Public Offering |  | $ 38.6 |  |  |
Maximum |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Dividend declared |  |  | $ 0.1 |  |
Repurchase expense |  |  | $ 0.4 | $ 0.4 | $ 0.5
Maximum | Restricted Stock |  |  |  |  |
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 |  |
Average | Dividend and Distribution Reinvestment and Share Purchase Plan |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Common stock price per share |  |  | $ 52.18 |  |

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