Equity - Additional Information (Detail) - USD ($)		12 Months Ended
	Jan. 30, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Common stock, shares outstanding		14,065,230	13,991,430
Common stock, shares authorized		25,000,000	25,000,000
Common stock, shares issued		32,095	36,265	38,020
Proceeds from Issuance of Common Stock		$ 1,300,000	$ 1,300,000	$ 1,200,000
Common stock shares repurchase		1,949	1,981	2,763
Repurchase expense		$ 100,000	$ 100,000	$ 100,000
Share based compensation expense		$ 2,200,000	1,900,000	1,400,000
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		$ 800,000	500,000
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]
Proceeds from Issuance of Common Stock		$ 1,300,000	$ 1,300,000	$ 1,200,000
Dividend and Distribution Reinvestment and Share Purchase Plan | Average
Class of Stock [Line Items]
Common stock price per share		$ 40.55
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock
Class of Stock [Line Items]
Common stock, shares issued		32,095	36,265	38,020
Unitil Energy Systems Inc | Series 6
Class of Stock [Line Items]
Preferred stock, outstanding		1,893	1,898
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		93,747	70,761
Restricted stock weighted average grant date fair value		$ 35.29	$ 32.56
Unrecognized share based compensation		$ 1,100,000
Share compensation recognition period		2 years 4 months 24 days
Forfeitures or cancellations under the stock plan		2,315
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%
Subsequent Event | Restricted Stock
Class of Stock [Line Items]
Shares	34,930
Aggregate Market Value	$ 1,600,000