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Details on Long Term Debt (Detail) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017 | Jun. 30, 2017
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | $ 396.0 | $ 409.4 | $ 336.2
Less: Unamortized Debt Issuance Costs | 3.2 | 3.3 | 2.8
Long-Term Debt | 392.8 | 406.1 | 333.4
Less: Current Portion | 29.7 | 29.8 | 29.9
Total Long-term Debt, Less Current Portion | 363.1 | 376.3 | 303.5
Long-Term Debt | 392.8 | 406.1 | 333.4
6.33% Senior Notes, Due May 1, 2022 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 20.0 | 20.0 | 20.0
3.70% Senior 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% Senior Secured Notes, Due March 2, 2020 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 10.0 | 15.0 | 15.0
Unitil Energy Systems Inc | First Mortgage Bonds 8.49% Senior Secured Notes, Due October 14, 2024 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 7.5 | 7.5 | 9.0
Unitil Energy Systems Inc | First Mortgage Bonds 6.96% Senior Secured Notes, 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% Senior Secured Notes, 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% Senior Secured Notes, 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% Senior Notes, Due November 30, 2023 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 7.6 | 7.6 | 9.5
Fitchburg Gas and Electric Light Company | 6.79% Senior 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 | 3.52% Senior Notes, Due November 1, 2027 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 10.0 | 10.0 |
Fitchburg Gas and Electric Light Company | 7.37% Senior 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% Senior 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% Senior Notes, Due June 1, 2031 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 14.0 | 14.0 | 14.0
Fitchburg Gas and Electric Light Company | 4.32% Senior Notes, Due November 1, 2047 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 15.0 | 15.0 |
Northern Utilities Inc | 3.52% Senior Notes, Due November 1, 2027 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 20.0 | 20.0 |
Northern Utilities Inc | 4.32% Senior Notes, Due November 1, 2047 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 30.0 | 30.0 |
Northern Utilities Inc | 6.95% Senior Notes, Due December 3, 2018 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 10.0 | 10.0 | 20.0
Northern Utilities Inc | 5.29% Senior Notes, Due March 2, 2020 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 16.6 | 25.0 | 25.0
Northern Utilities Inc | 7.72% Senior Notes, 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 | 3.3 | 3.3 | $ 6.7
Granite State Gas Transmission Inc | 3.72% Senior Notes, Due November 1, 2027 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | $ 15.0 | $ 15.0 |

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