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Common Stock And Preferred Stock - Additional Information (Detail) - USD ($)			3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 06, 2021	Jan. 26, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Class of Stock [Line Items]
Common stock, shares outstanding			15,971,962	15,005,881	15,971,962	15,005,881	15,012,310
Common stock, shares issued			925,493	6,650	936,512	16,849
Proceeds from Issuance of Common Stock					$ 45,300,000	$ 800,000
Restricted Stock Units Granted					$ 49.61	$ 54.41
Share based compensation expense					$ 1,400,000	$ 2,100,000
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			$ 800,000	$ 600,000	$ 800,000	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	200,000	$ 200,000
Maximum
Class of Stock [Line Items]
Dividend declared			$ 100,000	$ 100,000	$ 100,000	$ 100,000
Restricted Stock
Class of Stock [Line Items]
Restricted stock vesting period					4 years
Restricted stock non-vested			57,408	59,813	57,408	59,813
Unrecognized share based compensation			$ 700,000		$ 700,000
Share compensation recognition period					2 years 8 months 12 days
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		677,500
Restricted stock that may be awarded in any one calendar year to any one participant			20,000		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				39,057		39,057
Weighted-Average Stock Price				$ 41.54		$ 41.54
Series 6 | Unitil Energy Systems Inc
Class of Stock [Line Items]
Preferred stock, outstanding			1,861	1,887	1,861	1,887	1,887
Preferred Stock			$ 200,000	$ 200,000	$ 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					$ 801,200
Dividend and Distribution Reinvestment and Share Purchase Plan | Average
Class of Stock [Line Items]
Common stock price per share				$ 48.52		$ 48.52
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock
Class of Stock [Line Items]
Common stock, shares issued					16,512
Over-Allotment Option [Member]
Class of Stock [Line Items]
Number of shares granted over-allotment 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
Common Stock Offering [Member]
Class of Stock [Line Items]
Common stock, shares issued	800,000
Common stock price per share	$ 50.80
Proceeds from Issuance Initial Public Offering	$ 38,600,000

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