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Details on Long Term Debt (Parenthetical) (Detail) | Mar. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2012
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Senior Notes Six Point Three Three Percent Due May One Twenty Twenty Two |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 6.33% | 6.33% | 6.33%
Unitil Energy Systems Inc | First Mortgage Bonds Five Point Two Four Percent Due March Two Twenty Twenty |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 5.24% | 5.24% | 5.24%
Unitil Energy Systems Inc | First Mortgage Bonds Eight Point Four Nine Percent Due October Fourteen Twenty Twenty Four |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 8.49% | 8.49% | 8.49%
Unitil Energy Systems Inc | First Mortgage Bonds Six Point Nine Six Percent Due September One Twenty Twenty Eight |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 6.96% | 6.96% | 6.96%
Unitil Energy Systems Inc | First Mortgage Bonds Eight Point Zero Zero Percent Due May One Twenty Thirty One |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 8.00% | 8.00% | 8.00%
Unitil Energy Systems Inc | First Mortgage Bonds Six Point Three Two Percent Due September Fifteen Twenty Thirty Six |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 6.32% | 6.32% | 6.32%
Fitchburg Gas and Electric Light Company | Notes Six Point Seven Five Percent Due November Thirty Twenty Twenty Three |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 6.75% | 6.75% | 6.75%
Fitchburg Gas and Electric Light Company | Notes Seven Point Three Seven Percent Due January Fifteen Twenty Twenty Nine |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 7.37% | 7.37% | 7.37%
Fitchburg Gas and Electric Light Company | Notes Seven Point Nine Eight Percent Due June One Twenty Thirty One |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 7.98% | 7.98% | 7.98%
Fitchburg Gas and Electric Light Company | Notes Six Point Seven Nine Percent Due October Fifteen Twenty Twenty Five |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 6.79% | 6.79% | 6.79%
Fitchburg Gas and Electric Light Company | Notes Five Point Nine Zero Percent Due December Fifteen Twenty Thirty |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 5.90% | 5.90% | 5.90%
Northern Utilities Inc | Senior Notes Six Point Nine Five Percent Due December Three Twenty Eighteen |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 6.95% | 6.95% | 6.95%
Northern Utilities Inc | Senior Notes Five Point Two Nine Percent Due March Two Twenty Twenty |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 5.29% | 5.29% | 5.29%
Northern Utilities Inc | Senior Notes Seven Point Seven Two Percent Due December Three Twenty Thirty Eight |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 7.72% | 7.72% | 7.72%
Granite State Gas Transmission Inc | Senior Notes Seven Point One Five Percent Due December Fifteen Twenty Eighteen |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 7.15% | 7.15% | 7.15%
Unitil Realty Corp | Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate | 8.00% | 8.00% | 8.00%

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- Definition Interest rate stated in the contractual debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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