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Details on Long Term Debt (Detail) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018 | Mar. 31, 2018
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | $ 395.9 | $ 409.3 | $ 396.0
Less: Unamortized Debt Issuance Costs | 3.4 | 3.5 | 3.2
Long-Term Debt | 392.5 | 405.8 | 392.8
Less: Current Portion | 19.5 | 18.4 | 29.8
Total Long-Term Debt, Less Current Portion | 373.0 | 387.4 | 363.0
Long-Term Debt | 392.5 | 405.8 | 392.8
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 | 5.0 | 10.0 | 10.0
Unitil Energy Systems Inc | First Mortgage Bonds 8.49% Senior Secured Notes, Due October 14, 2024 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 6.0 | 6.0 | 7.5
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
Unitil Energy Systems Inc | First Mortgage Bonds 4.18% Senior Secured Notes Due November 30, 2048 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 30.0 | 30.0 |
Fitchburg Gas and Electric Light Company | 6.75% Senior Notes, Due November 30, 2023 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 5.7 | 5.7 | 7.6
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 | 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
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 | 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 | 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 | 30.0
Northern Utilities Inc | 6.95% Senior Notes, Due December 3, 2018 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  |  | 10.0
Northern Utilities Inc | 5.29% Senior Notes, Due March 2, 2020 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 8.2 | 16.6 | 16.6
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
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 | $ 15.0

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- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. 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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -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://fasb.org/us-gaap/role/ref/legacyRef -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 + Details
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