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Common Stock and Preferred Stock - Additional Information (Detail) (USD $) | 3 Months Ended | 6 Months Ended |  | 3 Months Ended | 6 Months Ended |  |  |  | 6 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 1 Months Ended | 6 Months Ended |  |  |
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Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012 | Dec. 31, 2012 | Jun. 30, 2013 Maximum | Jun. 30, 2012 Maximum | Jun. 30, 2013 Maximum | Jun. 30, 2012 Maximum | Jun. 30, 2012 Restricted Stock Unit | Jun. 30, 2013 Restricted Stock Unit Maximum | Dec. 31, 2012 Restricted Stock Unit Maximum | Jun. 30, 2013 Restricted Stock Y | Jun. 30, 2012 Restricted Stock | Jun. 30, 2013 Restricted Stock Maximum | Dec. 31, 2012 Series 8 Fitchburg Gas and Electric Light Company | Jun. 30, 2012 Series 8 Fitchburg Gas and Electric Light Company | Dec. 31, 2012 Series 5 Fitchburg Gas and Electric Light Company | Jun. 30, 2012 Series 5 Fitchburg Gas and Electric Light Company | May 16, 2012 Common Stock Offering | Jun. 30, 2013 Dividend and Distribution Reinvestment and Share Purchase Plan | Jun. 30, 2013 Restricted Stock Units Issued | Jun. 30, 2013 Series 6 Unitil Energy Systems Inc | Dec. 31, 2012 Series 6 Unitil Energy Systems Inc | Jun. 30, 2012 Series 6 Unitil Energy Systems Inc
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares outstanding | 13,758,805 | 13,822,318 | 13,758,805 | 13,780,601 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, share issued |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,760,000 | 20,477 |  |  |  |
Common stock price per share |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 25.25 | $ 28.27 |  |  |  |
Proceed from issuance of common stock |  | $ 600,000 | $ 66,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 65,700,000 | $ 579,000 |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  | 500,000 | 1,000,000 |  |  |  |  |  |  |  |  |  |  |
Unrecognized share based compensation |  |  |  |  |  |  |  |  |  |  |  | 900,000 |  |  |  |  |  |  |  |  |  |  |  |
Share compensation recognition period |  |  |  |  |  |  |  |  |  |  |  | 2.7 |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,654 |  | 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 | $ 100,000 | $ 100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition The aggregate amount to be paid by the entity upon redemption of the security that is classified as temporary equity. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. If convertible, the issuer does not control the actions or events necessary to issue the maximum number of shares that could be required to be delivered under the conversion option if the holder exercises the option to convert the stock to another class of equity. If the security is a warrant or a rights issue, the warrant or rights issue is considered to be temporary equity if the issuer cannot demonstrate that it would be able to deliver upon the exercise of the option by the holder in all cases. Includes stock with put option held by ESOP and stock redeemable by holder only in the event of a change in control of the issuer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.27(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 28 -Subparagraph b -Article 5 + Details
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