Equity - Additional Information (Detail) - USD ($)		12 Months Ended
	Jan. 26, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Common stock, shares outstanding		15,012,310	14,930,170
Common stock, shares issued		23,658	20,065	25,932
Common stock, shares authorized		25,000,000	25,000,000
Proceeds from Issuance of Common Stock		$ 1,100,000	$ 1,100,000	$ 1,200,000
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled		70.00%
Common stock shares repurchase		13,194	2,911	791
Fair value of liabilities associated with fully vested RSUs that will be settled in cash		$ 800,000	$ 1,900,000
Share based compensation expense		$ 2,200,000	2,300,000	$ 2,200,000
Percentage of fully-vested restricted stock units that directors will receive in cash when settled		30.00%
Preferred Stock		$ 200,000	$ 200,000
Restricted Stock
Class of Stock [Line Items]
Restricted stock vesting period		4 years
Restricted stock non-vested		39,426	32,950
Restricted stock weighted average grant date fair value		$ 55.46	$ 47.35
Unrecognized share based compensation		$ 800,000
Share compensation recognition period		2 years 8 months 12 days
Forfeitures under the stock plan		5,570
Cancellations under the stock plan		0
Restricted Stock | Subsequent Event
Class of Stock [Line Items]
Restricted Stock Units Granted	23,140
Aggregate Market Value	$ 900,000
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		1,887	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		$ 1,100,000	$ 1,100,000	$ 1,200,000
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock
Class of Stock [Line Items]
Common stock, shares issued		23,658	20,065	25,932
Maximum
Class of Stock [Line Items]
Dividend declared		$ 100,000	$ 100,000
Repurchase expense		$ 500,000	$ 200,000	$ 100,000
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		$ 46.08