Name:	us-gaap_ClassOfStockLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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.

Common Stock and Preferred Stock - Additional Information (Detail) (USD $)	6 Months Ended			3 Months Ended		6 Months Ended							6 Months Ended		12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Maximum	Jun. 30, 2013 Maximum	Jun. 30, 2014 Maximum	Jun. 30, 2013 Maximum	Jun. 30, 2014 Dividend and Distribution Reinvestment and Share Purchase Plan	Jun. 30, 2014 Restricted Stock	Jun. 30, 2013 Restricted Stock	Jun. 30, 2014 Restricted Stock Maximum	Jun. 30, 2014 Restricted Stock Units (RSUs)	Jun. 30, 2014 Unitil Energy Systems Inc Series 6	Jun. 30, 2013 Unitil Energy Systems Inc Series 6	Dec. 31, 2013 Unitil Energy Systems Inc Series 6
Class of Stock [Line Items]
Common stock, shares outstanding	13,895,777	13,822,318	13,841,400
Common stock, share issued								18,877
Common stock price per share								$ 32.07
Proceed from issuance of common stock	$ 600,000	$ 600,000						$ 605,000
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 period									4 years
Restricted stock vesting percentage annually									25.00%
Restricted stock issue date									Jan. 31, 2014
Restricted stock issued	35,500
Restricted stock grant date market value	1,100,000
Restricted stock non-vested									67,334	53,480
Restricted stock weighted average grant date fair value									$ 28.51	$ 25.99
Share based compensation expense	1,200,000	500,000
Unrecognized share based compensation									1,000,000
Share compensation recognition period									2 years 9 months 18 days
Forfeitures or cancellations under the stock plan									0
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		100,000		100,000
Weighted-Average Stock Price												$ 27.42
Restricted stock units outstanding												3,977
Preferred stock, outstanding													2,250	2,250	2,250
Preferred Stock	200,000	200,000	200,000										200,000	200,000	200,000
Dividend rate													6.00%	6.00%	6.00%
Dividend declared				$ 100,000	$ 100,000	$ 100,000	$ 100,000