Common Stock And Preferred Stock - Additional Information (Detail) - USD ($)			3 Months Ended		12 Months Ended
	Jan. 29, 2018	Dec. 14, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Class of Stock [Line Items]
Common stock, shares outstanding			14,860,123	14,107,741	14,815,585
Common stock, shares issued			7,812	7,581
Proceeds from Issuance of Common Stock			$ 300,000	$ 300,000
Share based compensation expense			$ 1,800,000	1,700,000
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	$ 1,000,000
Preferred Stock			200,000	200,000	$ 200,000
Maximum
Class of Stock [Line Items]
Dividend declared			100,000	$ 100,000
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
Dividend and Distribution Reinvestment and Share Purchase Plan
Class of Stock [Line Items]
Proceeds from Issuance of Common Stock			$ 341,500
Dividend and Distribution Reinvestment and Share Purchase Plan | Average
Class of Stock [Line Items]
Common stock price per share			$ 43.71
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock
Class of Stock [Line Items]
Common stock, shares issued			7,812
Unitil Energy Systems Inc | Series 6
Class of Stock [Line Items]
Preferred stock, outstanding			1,893	1,893	1,893
Preferred Stock			$ 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			90,882	94,880
Restricted stock weighted average grant date fair value			$ 41.93	$ 39.55
Unrecognized share based compensation			$ 1,300,000
Share compensation recognition period			2 years 9 months 18 days
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
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%
Restricted Stock Units (RSUs)
Class of Stock [Line Items]
Restricted Stock Units Granted			0
Restricted stock units outstanding				43,696
Weighted-Average Stock Price				$ 33.49