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Details on Preferred Stock (Parenthetical) (Detail) (USD $) | 3 Months Ended | 12 Months Ended | 3 Months Ended
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Mar. 31, 2013 Unitil Energy Systems Inc Nonredeemable Preferred Stock Noncumulative Preferred Stock Series 6 | Dec. 31, 2012 Unitil Energy Systems Inc Nonredeemable Preferred Stock Noncumulative Preferred Stock Series 6 | Mar. 31, 2012 Unitil Energy Systems Inc Nonredeemable Preferred Stock Noncumulative Preferred Stock Series 6 | Dec. 31, 2012 Fitchburg Gas and Electric Light Company Series 5 | Dec. 31, 2012 Fitchburg Gas and Electric Light Company Series 8 | Mar. 31, 2013 Fitchburg Gas and Electric Light Company Redeemable Preferred Stock Cumulative Preferred Stock Series 5 | Dec. 31, 2012 Fitchburg Gas and Electric Light Company Redeemable Preferred Stock Cumulative Preferred Stock Series 5 | Mar. 31, 2012 Fitchburg Gas and Electric Light Company Redeemable Preferred Stock Cumulative Preferred Stock Series 5 | Mar. 31, 2013 Fitchburg Gas and Electric Light Company Redeemable Preferred Stock Cumulative Preferred Stock Series 8 | Dec. 31, 2012 Fitchburg Gas and Electric Light Company Redeemable Preferred Stock Cumulative Preferred Stock Series 8 | Mar. 31, 2012 Fitchburg Gas and Electric Light Company Redeemable Preferred Stock Cumulative Preferred Stock Series 8
Preferred Units [Line Items] |  |  |  |  |  |  |  |  |  |  |
Dividend rate | 6.00% | 6.00% | 6.00% | 5.125% | 8.00% | 5.125% | 5.125% | 5.125% | 8.00% | 8.00% | 8.00%
Preferred Stock Par Value | $ 100 | $ 100 | $ 100 |  |  | $ 100 | $ 100 | $ 100 | $ 100 | $ 100 | $ 100

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- Definition Face amount or stated value per share of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer); generally not indicative of the fair market value per share. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 129 -Paragraph 2, 3, 4, 5, 6, 7, 8 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 10 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 Reference 4: 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 + Details
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