Common Stock And Preferred Stock - Additional Information (Detail) - USD ($)				3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 01, 2018	Jan. 29, 2018	Dec. 14, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Class of Stock [Line Items]
Common stock, shares outstanding				14,872,011	14,119,893	14,872,011	14,119,893	14,815,585
Common stock, shares issued				5,423	6,173	19,700	20,564
Proceeds from Issuance of Common Stock						$ 900,000	$ 1,000,000
Share based compensation expense						$ 2,100,000	2,600,000
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled						70.00%
Percentage of fully-vested restricted stock units that directors will receive in cash when settled						30.00%
Fair value of liabilities associated with fully vested RSUs that will be settled in cash				$ 1,200,000	$ 900,000	$ 1,200,000	900,000	$ 1,000,000
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
Dividend and Distribution Reinvestment and Share Purchase Plan
Class of Stock [Line Items]
Proceeds from Issuance of Common Stock						$ 925,000
Dividend and Distribution Reinvestment and Share Purchase Plan | Average
Class of Stock [Line Items]
Common stock price per share				$ 46.97		$ 46.97
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock
Class of Stock [Line Items]
Common stock, shares issued						19,700
Registered Public Offering
Class of Stock [Line Items]
Proceeds from Issuance of Common Stock			$ 31,700,000
Registered Public Offering | Common Stock
Class of Stock [Line Items]
Common stock, shares issued			690,000
Common stock price per share			$ 48.30
Unitil Energy Systems Inc | Series 6
Class of Stock [Line Items]
Preferred stock, outstanding				1,893	1,893	1,893	1,893	1,893
Preferred Stock				$ 200,000	$ 200,000	$ 200,000	$ 200,000	$ 200,000
Dividend rate						6.00%	6.00%	6.00%
Restricted Stock
Class of Stock [Line Items]
Restricted stock vesting period						4 years
Restricted stock non-vested				49,581	89,326	49,581	89,326
Restricted stock weighted average grant date fair value						$ 41.96	$ 39.54
Unrecognized share based compensation				$ 1,000,000		$ 1,000,000
Share compensation recognition period						2 years 6 months
Restricted Stock Units Granted		37,510
Aggregate Market Value		$ 1,600,000
Forfeitures under the stock plan						784
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	11,275					0
Restricted stock units outstanding					44,343		44,343
Weighted-Average Stock Price					$ 33.72		$ 33.72