Equity - Additional Information (Detail) - USD ($)			12 Months Ended
	Jan. 29, 2018	Dec. 14, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Common stock, shares outstanding			14,815,585	14,065,230
Common stock, shares authorized			25,000,000	25,000,000
Common stock, shares issued				32,095	36,265
Proceeds from Issuance of Common Stock			$ 33,000,000	$ 1,300,000	$ 1,300,000
Common stock shares repurchase			1,686	1,949	1,981
Repurchase expense			$ 100,000	$ 100,000	$ 100,000
Share based compensation expense			$ 2,700,000	2,200,000	1,900,000
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			$ 1,000,000	800,000
Preferred Stock			200,000	200,000
Maximum
Class of Stock [Line Items]
Dividend declared			$ 100,000	100,000
Dividend and Distribution Reinvestment and Share Purchase Plan
Class of Stock [Line Items]
Common stock, shares issued			26,256
Proceeds from Issuance of Common Stock			$ 1,300,000	$ 1,300,000	$ 1,300,000
Dividend and Distribution Reinvestment and Share Purchase Plan | Average
Class of Stock [Line Items]
Common stock price per share			$ 47.71
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock
Class of Stock [Line Items]
Common stock, shares issued			26,256	32,095	36,265
Registered Public Offering
Class of Stock [Line Items]
Common stock, shares issued			690,000
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
Preferred Stock			$ 200,000	$ 200,000
Dividend rate			6.00%	6.00%
Restricted Stock
Class of Stock [Line Items]
Restricted stock vesting period			4 years
Restricted stock non-vested			89,326	93,747
Restricted stock weighted average grant date fair value			$ 39.54	$ 35.29
Unrecognized share based compensation			$ 800,000
Share compensation recognition period			2 years 3 months 19 days
Forfeitures under the stock plan			831
Cancellations under the stock plan			0
Restricted Stock | Subsequent Event
Class of Stock [Line Items]
Restricted Stock Units Granted	37,510
Aggregate Market Value	$ 1,600,000
Restricted Stock | Maximum
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
Restricted Stock | Vesting Annually
Class of Stock [Line Items]
Restricted stock vesting percentage annually			25.00%