Common Stock And Preferred Stock - Additional Information (Detail) - USD ($)			3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 01, 2019	Jan. 29, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Class of Stock [Line Items]
Common stock, shares outstanding			14,925,898	14,872,011	14,925,898	14,872,011	14,876,955
Common stock, shares issued			4,727	5,423	15,793	19,700
Proceeds from Issuance of Common Stock					$ 900,000	$ 900,000
Restricted stock weighted average grant date fair value					$ 46.41	$ 41.96
Share based compensation expense					$ 2,200,000	$ 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			$ 1,700,000	$ 1,200,000	$ 1,700,000	1,200,000	$ 1,300,000
Percentage of fully-vested restricted stock units that directors will receive in cash when settled					30.00%
Preferred Stock			200,000	200,000	$ 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			50,917	49,581	50,917	49,581
Unrecognized share based compensation			$ 800,000		$ 800,000
Share compensation recognition period					2 years 8 months 12 days
Restricted Stock Units Granted		33,150
Aggregate Market Value					$ 1,600,000
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	9,918				0
Restricted stock units outstanding				53,454		53,454
Weighted-Average Stock Price				$ 36.47		$ 36.47
Series 6 | Unitil Energy Systems Inc
Class of Stock [Line Items]
Preferred stock, outstanding			1,887	1,893	1,887	1,893	1,893
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					$ 889,000
Dividend and Distribution Reinvestment and Share Purchase Plan | Average
Class of Stock [Line Items]
Common stock price per share			$ 56.29		$ 56.29
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock
Class of Stock [Line Items]
Common stock, shares issued					15,793