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DEBT AND FINANCING ARRANGEMENTS - Details On Long Term Debt (Detail) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | $ 516.0 | $ 517.2 | $ 497.9
Less: Unamortized Debt Issuance Costs | 3.2 | 3.2 | 3.2
Long-Term Debt | 512.8 | 514.0 | 494.7
Less: Current Portion | 4.9 | 4.9 | 6.7
Total Long-Term Debt, Less Current Portion | 507.9 | 509.1 | 488.0
3.70% Senior Notes, Due August 1, 2026 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 30.0 | 30.0 | 30.0
3.43% Senior Notes, Due December 18, 2029 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 30.0 | 30.0 | 30.0
Unitil Energy Systems Inc | First Mortgage Bonds 6.96% Senior Secured Notes, Due September 1, 2028 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 10.0 | 10.0 | 12.0
Unitil Energy Systems Inc | First Mortgage Bonds 8.00% Senior Secured Notes, Due May 1, 2031 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 12.0 | 12.0 | 13.5
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 3.58% Senior Secured Notes, Due September 15, 2040 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 27.5 | 27.5 | 27.5
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 | 30.0
Fitchburg Gas and Electric Light Company | 6.79% Senior Notes, Due October 15, 2025 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 0.0 | 0.0 | 2.0
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 | 7.98% 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 | 7.37% Notes, Due January 15, 2029 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 6.0 | 7.2 | 7.2
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 | 5.70% Notes, Due July 2, 2033 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 12.0 | 12.0 | 0.0
Fitchburg Gas and Electric Light Company | 5.96% Senior Notes, Due July 2, 2053 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 13.0 | 13.0 | 0.0
Fitchburg Gas and Electric Light Company | 3.78% Senior Notes, Due September 15, 2040 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 27.5 | 27.5 | 27.5
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 | 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 | 3.78% Senior Notes, Due September 15, 2040 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 40.0 | 40.0 | 40.0
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
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 | 4.04% Senior Notes, Due September 12, 2049 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | 40.0 | 40.0 | 40.0
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
Unitil Realty Corp [Member] | 2.64% Senior Secured Notes, Due December 18, 2030 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt | $ 4.0 | $ 4.0 | $ 4.2

X
- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -SubTopic 10 -Topic 210 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481139/470-20-50-4 + Details
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