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 $)	9 Months Ended			3 Months Ended		9 Months Ended				1 Months Ended		9 Months Ended				12 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Maximum	Sep. 30, 2012 Maximum	Sep. 30, 2013 Maximum	Sep. 30, 2012 Restricted Stock Units	Sep. 30, 2013 Restricted Stock Units Maximum	Dec. 31, 2012 Restricted Stock Units Maximum	Oct. 01, 2013 Restricted Stock Units Subsequent Event	May 16, 2012 Common Stock Offering	Sep. 30, 2013 Dividend and Distribution Reinvestment and Share Purchase Plan	Sep. 30, 2013 Restricted Stock Y	Sep. 30, 2012 Restricted Stock	Sep. 30, 2013 Restricted Stock Maximum	Dec. 31, 2012 Series 8 Fitchburg Gas and Electric Light Company	Sep. 30, 2012 Series 8 Fitchburg Gas and Electric Light Company	Dec. 31, 2012 Series 5 Fitchburg Gas and Electric Light Company	Sep. 30, 2012 Series 5 Fitchburg Gas and Electric Light Company	Sep. 30, 2013 Series 6 Unitil Energy Systems Inc	Dec. 31, 2012 Series 6 Unitil Energy Systems Inc	Sep. 30, 2012 Series 6 Unitil Energy Systems Inc
Class of Stock [Line Items]
Common stock, shares outstanding	13,831,624	13,769,376	13,780,601
Common stock, share issued											2,760,000	29,783
Common stock price per share											$ 25.25	$ 28.73
Proceed from issuance of common stock	$ 900,000	$ 66,500,000									$ 65,700,000	$ 856,000
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													Feb. 04, 2013
Restricted stock issued													21,240
Restricted stock grant date market value													564,134
Restricted stock non-vested													53,480	53,942
Restricted stock weighted average grant date fair value													$ 25.99	$ 24.67
Share based compensation expense													600,000	1,100,000
Unrecognized share based compensation													800,000
Share compensation recognition period													2.6
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%
Restricted stock units outstanding							0
Fully-vested restricted stock units issued										15,300
Fair value of liabilities associated with fully vested RSUs that will be settled in cash							0	100,000	100,000
Preferred stock, outstanding																	9,654		7,823	2,250	2,250	2,250
Redeemable, Cumulative Preferred Stock																965,400		792,313
Dividend rate																8.00%		5.125%
Preferred Stock premium percentage																		1.28%
Dividend declared		$ 100,000		$ 100,000	$ 100,000	$ 100,000