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Details on Long Term Debt (Detail) - USD ($) $ in Millions | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | ---
Debt Instrument [Line Items] |  |
Long-Term Debt | $ 325.5 | $ 332.9
Less: Current Portion | 17.4 | 4.0
Total Long-term Debt, Less Current Portion | 308.1 | 328.9
Long-Term Debt | 325.5 | 332.9
6.33% Notes, Due May 1, 2022 |  |
Debt Instrument [Line Items] |  |
Senior Notes | 20.0 | 20.0
Unitil Energy Systems Inc | First Mortgage Bonds 5.24% Series, Due March 2, 2020 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 15.0 | 15.0
Long-Term Debt | 15.0 | 15.0
Unitil Energy Systems Inc | First Mortgage Bonds 8.49% Series, Due October 14, 2024 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 12.0 | 15.0
Long-Term Debt | 12.0 | 15.0
Unitil Energy Systems Inc | First Mortgage Bonds 6.96% Series, Due September 1, 2028 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 20.0 | 20.0
Long-Term Debt | 20.0 | 20.0
Unitil Energy Systems Inc | First Mortgage Bonds 8.00% Series, Due May 1, 2031 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 15.0 | 15.0
Long-Term Debt | 15.0 | 15.0
Unitil Energy Systems Inc | First Mortgage Bonds 6.32% Series, Due September 15, 2036 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 15.0 | 15.0
Long-Term Debt | 15.0 | 15.0
Fitchburg Gas and Electric Light Company | 6.75% Notes, Due November 30, 2023 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 11.4 | 15.2
Long-Term Debt | 11.4 | 15.2
Fitchburg Gas and Electric Light Company | 7.37% Notes, Due January 15, 2029 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 12.0 | 12.0
Long-Term Debt | 12.0 | 12.0
Fitchburg Gas and Electric Light Company | 7.98% Notes, Due June 1, 2031 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 14.0 | 14.0
Long-Term Debt | 14.0 | 14.0
Fitchburg Gas and Electric Light Company | 6.79% Notes, Due October 15, 2025 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 10.0 | 10.0
Long-Term Debt | 10.0 | 10.0
Fitchburg Gas and Electric Light Company | 5.90% Notes, Due December 15, 2030 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 15.0 | 15.0
Long-Term Debt | 15.0 | 15.0
Northern Utilities Inc | 6.95% Senior Notes, Series A, Due December 3, 2018 |  |
Debt Instrument [Line Items] |  |
Senior Notes | 30.0 | 30.0
Northern Utilities Inc | 5.29% Senior Notes, Due March 2, 2020 |  |
Debt Instrument [Line Items] |  |
Senior Notes | 25.0 | 25.0
Northern Utilities Inc | 7.72% Senior Notes, Series B, Due December 3, 2038 |  |
Debt Instrument [Line Items] |  |
Senior Notes | 50.0 | 50.0
Northern Utilities Inc | 4.42% Senior Notes, Due October 15, 2044 |  |
Debt Instrument [Line Items] |  |
Senior Notes | 50.0 | 50.0
Granite State Gas Transmission Inc | 7.15% Senior Notes, Due December 15, 2018 |  |
Debt Instrument [Line Items] |  |
Senior Notes | 10.0 | 10.0
Unitil Realty Corp | 8.00% Senior Secured Notes, Due August 1, 2017 |  |
Debt Instrument [Line Items] |  |
Senior Secured Notes | $ 1.1 | $ 1.7

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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 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 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 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 + Details
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