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 $)	3 Months Ended			3 Months Ended					3 Months Ended			12 Months Ended		12 Months Ended		1 Months Ended	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Maximum	Mar. 31, 2012 Maximum	Mar. 31, 2012 Restricted Stock Unit	Mar. 31, 2013 Restricted Stock Unit Maximum	Dec. 31, 2012 Restricted Stock Unit Maximum	Mar. 31, 2013 Restricted Stock Y	Mar. 31, 2012 Restricted Stock	Mar. 31, 2013 Restricted Stock Maximum	Dec. 31, 2012 Series 8 Fitchburg Gas and Electric Light Company	Mar. 31, 2012 Series 8 Fitchburg Gas and Electric Light Company	Dec. 31, 2012 Series 5 Fitchburg Gas and Electric Light Company	Mar. 31, 2012 Series 5 Fitchburg Gas and Electric Light Company	May 16, 2012 Common Stock Offering	Mar. 31, 2013 Dividend and Distribution Reinvestment and Share Purchase Plan	Mar. 31, 2013 Restricted Stock Units Issued	Mar. 31, 2013 Series 6 Unitil Energy Systems Inc	Dec. 31, 2012 Series 6 Unitil Energy Systems Inc	Mar. 31, 2012 Series 6 Unitil Energy Systems Inc
Class of Stock [Line Items]
Common stock, shares outstanding	13,812,763	10,989,389	13,780,601
Common stock, share issued																2,760,000	10,922
Common stock price per share																$ 25.25	$ 27.01
Proceed from issuance of common stock																$ 65,700,000	$ 295,000
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	57,638
Restricted stock weighted average grant date fair value									$ 25.99	$ 24.68
Share based compensation expense									400,000	300,000
Unrecognized share based compensation									1,000,000
Share compensation recognition period									3.0
Forfeitures or cancellations under the stock plan									0
Restricted stock units issued																		0
Fair value of liabilities associated with fully vested RSUs that will be settled in cash						0	100,000	100,000
Preferred stock, outstanding													9,696		7,861				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