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Details on Long Term Debt (Detail) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016 | Sep. 30, 2016
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | $ 336.2 | $ 336.6 | $ 355.0
Less: Unamortized Debt Issuance Costs | 2.8 | 3.0 | 3.0
Long-Term Debt | 333.4 | 333.6 | 352.0
Less: Current Portion | 29.8 | 16.8 | 17.0
Total Long-Term Debt, Less Current Portion | 303.6 | 316.8 | 335.0
Long-Term Debt | 333.4 | 333.6 | 352.0
6.33% Notes, Due May 1, 2022 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 20.0 | 20.0 | 20.0
3.70% Senior Unsecured Notes due August 1, 2026 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 30.0 | 30.0 | 30.0
Unitil Energy Systems Inc | First Mortgage Bonds 5.24% Series, Due March 2, 2020 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 15.0 | 15.0 | 15.0
Unitil Energy Systems Inc | First Mortgage Bonds 8.49% Series, Due October 14, 2024 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 9.0 | 9.0 | 12.0
Unitil Energy Systems Inc | First Mortgage Bonds 6.96% Series, Due September 1, 2028 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 20.0 | 20.0 | 20.0
Unitil Energy Systems Inc | First Mortgage Bonds 8.00% Series, Due May 1, 2031 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 15.0 | 15.0 | 15.0
Unitil Energy Systems Inc | First Mortgage Bonds 6.32% Series, Due September 15, 2036 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 15.0 | 15.0 | 15.0
Fitchburg Gas and Electric Light Company | 6.75% Notes, Due November 30, 2023 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 9.5 | 9.5 | 11.4
Fitchburg Gas and Electric Light Company | 6.79% Notes, Due October 15, 2025 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 10.0 | 10.0 | 10.0
Fitchburg Gas and Electric Light Company | 7.37% Notes, Due January 15, 2029 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 12.0 | 12.0 | 12.0
Fitchburg Gas and Electric Light Company | 5.90% Notes, Due December 15, 2030 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 15.0 | 15.0 | 15.0
Fitchburg Gas and Electric Light Company | 7.98% Notes, Due June 1, 2031 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 14.0 | 14.0 | 14.0
Northern Utilities Inc | 6.95% Senior Notes, Series A, Due December 3, 2018 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 20.0 | 20.0 | 30.0
Northern Utilities Inc | 5.29% Senior Notes, Due March 2, 2020 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 25.0 | 25.0 | 25.0
Northern Utilities Inc | 7.72% Senior Notes, Series B, Due December 3, 2038 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 50.0 | 50.0 | 50.0
Northern Utilities Inc | 4.42% Senior Notes, Due October 15, 2044 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 50.0 | 50.0 | 50.0
Granite State Gas Transmission Inc | 7.15% Senior Notes, Due December 15, 2018 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | $ 6.7 | 6.7 | 10.0
Unitil Realty Corp | 8.00% Senior Secured Notes, Due August 1, 2017 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | $ 0.4 | $ 0.6

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