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Details on Long Term Debt (Detail) (USD $) In Millions, unless otherwise specified | Sep. 30, 2013 | Dec. 31, 2012 | Sep. 30, 2012
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Long-Term Debt | $ 287.5 | $ 287.8 | $ 288.0
Less: Current Portion | 0.6 | 0.5 | 0.5
Total Long-term Debt, Less Current Portion | 286.9 | 287.3 | 287.5
Senior Notes Six Point Three Three Percent Due May One Twenty Twenty Two |  |  |
Debt Instrument [Line Items] |  |  |
Senior Notes | 20.0 | 20.0 | 20.0
Unitil Energy Systems Inc | First Mortgage Bonds Five Point Two Four Percent Due March Two Twenty Twenty |  |  |
Debt Instrument [Line Items] |  |  |
Long-Term Debt | 15.0 | 15.0 | 15.0
Unitil Energy Systems Inc | First Mortgage Bonds Eight Point Four Nine Percent Due October Fourteen Twenty Twenty Four |  |  |
Debt Instrument [Line Items] |  |  |
Long-Term Debt | 15.0 | 15.0 | 15.0
Unitil Energy Systems Inc | First Mortgage Bonds Six Point Nine Six Percent Due September One Twenty Twenty Eight |  |  |
Debt Instrument [Line Items] |  |  |
Long-Term Debt | 20.0 | 20.0 | 20.0
Unitil Energy Systems Inc | First Mortgage Bonds Eight Point Zero Zero Percent Due May One Twenty Thirty One |  |  |
Debt Instrument [Line Items] |  |  |
Long-Term Debt | 15.0 | 15.0 | 15.0
Unitil Energy Systems Inc | First Mortgage Bonds Six Point Three Two Percent Due September Fifteen Twenty Thirty Six |  |  |
Debt Instrument [Line Items] |  |  |
Long-Term Debt | 15.0 | 15.0 | 15.0
Fitchburg Gas and Electric Light Company | Notes Six Point Seven Five Percent Due November Thirty Twenty Twenty Three |  |  |
Debt Instrument [Line Items] |  |  |
Long-Term Debt | 19.0 | 19.0 | 19.0
Fitchburg Gas and Electric Light Company | Notes Seven Point Three Seven Percent Due January Fifteen Twenty Twenty Nine |  |  |
Debt Instrument [Line Items] |  |  |
Long-Term Debt | 12.0 | 12.0 | 12.0
Fitchburg Gas and Electric Light Company | Notes Seven Point Nine Eight Percent Due June One Twenty Thirty One |  |  |
Debt Instrument [Line Items] |  |  |
Long-Term Debt | 14.0 | 14.0 | 14.0
Fitchburg Gas and Electric Light Company | Notes Six Point Seven Nine Percent Due October Fifteen Twenty Twenty Five |  |  |
Debt Instrument [Line Items] |  |  |
Long-Term Debt | 10.0 | 10.0 | 10.0
Fitchburg Gas and Electric Light Company | Notes Five Point Nine Zero Percent Due December Fifteen Twenty Thirty |  |  |
Debt Instrument [Line Items] |  |  |
Long-Term Debt | 15.0 | 15.0 | 15.0
Northern Utilities Inc | Senior Notes Six Point Nine Five Percent Due December Three Twenty Eighteen |  |  |
Debt Instrument [Line Items] |  |  |
Senior Notes | 30.0 | 30.0 | 30.0
Northern Utilities Inc | Senior Notes Five Point Two Nine Percent Due March Two Twenty Twenty |  |  |
Debt Instrument [Line Items] |  |  |
Senior Notes | 25.0 | 25.0 | 25.0
Northern Utilities Inc | Senior Notes Seven Point Seven Two Percent Due December Three Twenty Thirty Eight |  |  |
Debt Instrument [Line Items] |  |  |
Senior Notes | 50.0 | 50.0 | 50.0
Granite State Gas Transmission Inc | Senior Notes Seven Point One Five Percent Due December Fifteen Twenty Eighteen |  |  |
Debt Instrument [Line Items] |  |  |
Senior Notes | 10.0 | 10.0 | 10.0
Unitil Realty Corp | Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen |  |  |
Debt Instrument [Line Items] |  |  |
Senior Secured Notes | $ 2.5 | $ 2.8 | $ 3.0

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- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 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) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + 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 210.5-02.19,20) -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 20 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Article 5 + Details
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- Definition Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16(a)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 + Details
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