Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended		12 Months Ended
	Jan. 24, 2023	Aug. 06, 2021	Jan. 31, 2024	Jan. 30, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Class of Stock [Line Items]
Common Stock, Shares, Outstanding					16,116,724	16,043,355
Common stock, shares issued					21,321	18,853	942,316
Common stock, shares authorized					25,000,000	25,000,000
Proceeds from Issuance of Common Stock					$ 1.1	$ 1.0	$ 45.5
Restricted stock weighted average grant date fair value					$ 46.94	$ 46.45
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled					70.00%
Common stock shares repurchase					14,680	9,449	8,012
Share based compensation expense					$ 1.4	$ 2.1	$ 1.4
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					36,483	45,473
Fair value of liabilities associated with fully vested RSUs that will be settled in cash					$ 0.8	$ 1.0
Unrecognized share based compensation					$ 0.7
Share compensation recognition period					2 years 2 months 12 days
Aggregate Market Value				$ 1.1
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Aggregate Intrinsic Value				$ 1.1
Cancellations under the stock plan					0
Restricted Stock | Subsequent Event
Class of Stock [Line Items]
Restricted Stock Units Granted			22,680
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period			22,680
Restricted Stock | Vesting Annually
Class of Stock [Line Items]
Restricted stock vesting percentage annually					25.00%
Restricted Stock | Period One [Member]
Class of Stock [Line Items]
Restricted Stock Units Granted					23,140
Aggregate Market Value					$ 0.9
Share Issued Date					Jan. 26, 2021
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period					23,140
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Aggregate Intrinsic Value					$ 0.9
Restricted Stock | Period Two [Member]
Class of Stock [Line Items]
Restricted Stock Units Granted					36,770
Aggregate Market Value					$ 1.7
Share Issued Date					Jan. 25, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period					36,770
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Aggregate Intrinsic Value					$ 1.7
Restricted Stock | Period Three [Member]
Class of Stock [Line Items]
Restricted Stock Units Granted					18,770
Aggregate Market Value					$ 1.0
Share Issued Date					Jan. 24, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period					18,770
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Aggregate Intrinsic Value					$ 1.0
Performance Restricted Shares
Class of Stock [Line Items]
Restricted stock non-vested					18,770
Restricted stock weighted average grant date fair value					$ 51.83
Share based compensation expense					$ 0.5
Unrecognized share based compensation					$ 0.9
Share compensation recognition period					2 years
Restricted Stock Units Granted	18,770
Aggregate Market Value	$ 1.0
Share Issued Date	Jan. 24, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period	18,770
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Aggregate Intrinsic Value	$ 1.0
Performance Restricted Shares | Subsequent Event
Class of Stock [Line Items]
Restricted Stock Units Granted				22,680
Aggregate Market Value				$ 1.1
Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period				22,680
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Aggregate Intrinsic Value				$ 1.1
Common Stock
Class of Stock [Line Items]
Common stock, shares issued		800,000
Common stock price per share		$ 50.8
Proceeds from Issuance Initial Public Offering		$ 38.6
Unitil Energy Systems Inc | Series 6
Class of Stock [Line Items]
Preferred stock, outstanding					1,727	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					$ 1.1	$ 1.0	$ 1.0
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock
Class of Stock [Line Items]
Common stock, shares issued					21,321	18,853	22,316
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
Maximum
Class of Stock [Line Items]
Dividend declared					$ 0.1	$ 0.1
Repurchase expense					$ 0.6	$ 0.4	$ 0.4
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					$ 51.58