Common Stock and Preferred Stock - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Class of Stock [Line Items]
Common stock, shares outstanding	13,886,768	13,812,763	13,841,400
Proceed from issuance of common stock	$ 300,000	$ 300,000
Restricted stock issued	35,500
Restricted stock grant date market value	1,100,000
Share based compensation expense	1,100,000	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 cash portion of fully vested RSUs	200,000		200,000
Preferred Stock	200,000	200,000	200,000
Maximum
Class of Stock [Line Items]
Fair value of liabilities associated with cash portion of fully vested RSUs		100,000
Dividend declared	100,000	100,000
Dividend and Distribution Reinvestment and Share Purchase Plan
Class of Stock [Line Items]
Common stock, share issued	9,868
Common stock price per share	$ 31.39
Proceed from issuance of common stock	310,000
Restricted Stock
Class of Stock [Line Items]
Restricted stock vesting period	4 years
Restricted stock vesting percentage annually	25.00%
Restricted stock issue date	Jan. 31, 2014
Restricted stock non-vested	69,073	53,480
Restricted stock weighted average grant date fair value	$ 28.52	$ 25.99
Unrecognized share based compensation	1,100,000
Share compensation recognition period	3 years
Forfeitures or 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 Units (RSUs)
Class of Stock [Line Items]
Weighted-Average Stock Price		$ 27.39
Restricted stock units outstanding		3,932
Unitil Energy Systems Inc \| Series 6
Class of Stock [Line Items]
Preferred stock, outstanding	2,250	2,250	2,250
Preferred Stock	$ 200,000	$ 200,000	$ 200,000
Dividend rate	6.00%	6.00%	6.00%

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.