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Details on Preferred Stock (Parenthetical) (Detail) (USD $) | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2013 | Jun. 30, 2012 | Dec. 31, 2012
Unitil Energy Systems Inc | Noncumulative Preferred Stock | Series 6 | Nonredeemable Preferred Stock |  |  |
Preferred Units [Line Items] |  |  |
Dividend rate | 6.00% | 6.00% | 6.00%
Preferred Stock Par Value | $ 100 | $ 100 | $ 100
Fitchburg Gas and Electric Light Company | Series 5 |  |  |
Preferred Units [Line Items] |  |  |
Dividend rate |  |  | 5.125%
Fitchburg Gas and Electric Light Company | Series 8 |  |  |
Preferred Units [Line Items] |  |  |
Dividend rate |  |  | 8.00%
Fitchburg Gas and Electric Light Company | Cumulative Preferred Stock | Series 5 | Redeemable Preferred Stock |  |  |
Preferred Units [Line Items] |  |  |
Dividend rate | 5.125% | 5.125% | 5.125%
Preferred Stock Par Value | $ 100 | $ 100 | $ 100
Fitchburg Gas and Electric Light Company | Cumulative Preferred Stock | Series 8 | Redeemable Preferred Stock |  |  |
Preferred Units [Line Items] |  |  |
Dividend rate | 8.00% | 8.00% | 8.00%
Preferred Stock Par Value | $ 100 | $ 100 | $ 100

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- Definition Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option 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 210.5-02.28) -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 29 -Article 5 + Details
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