Equity - Additional Information (Detail) - USD ($)			12 Months Ended
	Jan. 25, 2022	Aug. 06, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Common stock, shares outstanding			15,977,766	15,012,310
Common stock, shares issued		800,000	942,316	23,658	20,065
Common stock, shares authorized			25,000,000	25,000,000
Common stock price per share		$ 50.80
Proceeds from Issuance of Common Stock			$ 45,500,000	$ 1,100,000	$ 1,100,000
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled			70.00%
Common stock shares repurchase			8,012	13,194	2,911
Fair value of liabilities associated with fully vested RSUs that will be settled in cash			$ 1,000,000.0	$ 800,000
Share based compensation expense			$ 1,400,000	2,200,000	$ 2,300,000
Percentage of fully-vested restricted stock units that directors will receive in cash when settled			30.00%
Preferred Stock			$ 200,000	$ 200,000
Proceeds from Issuance Initial Public Offering		$ 38,600,000
Restricted Stock
Class of Stock [Line Items]
Restricted stock vesting period			4 years
Restricted stock non-vested			37,621	39,426
Restricted stock weighted average grant date fair value			$ 49.72	$ 55.46
Unrecognized share based compensation			$ 600,000
Share compensation recognition period			2 years 6 months
Cancellations under the stock plan			0
Restricted Stock | Subsequent Event
Class of Stock [Line Items]
Restricted Stock Units Granted	36,770
Aggregate Market Value	$ 1,700,000
Restricted Stock | Vesting Annually
Class of Stock [Line Items]
Restricted stock vesting percentage annually				25.00%
Unitil Energy Systems Inc | Series 6
Class of Stock [Line Items]
Preferred stock, outstanding			1,861	1,887
Preferred Stock			$ 200,000	$ 200,000
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			$ 1,000,000.0	$ 1,100,000	$ 1,100,000
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock
Class of Stock [Line Items]
Common stock, shares issued			22,316	23,658	20,065
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		$ 5,900,000
Maximum
Class of Stock [Line Items]
Dividend declared			$ 100,000	$ 100,000
Repurchase expense			$ 400,000	$ 500,000	$ 200,000
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			$ 46.98