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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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- Definition 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. + 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 210.5-02.29) -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 30 -Article 5 + Details
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