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Debt and Financing Arrangements - Details on Long Term Debt (Detail) - USD ($)
$ in Millions
Dec. 31, 2024
Dec. 31, 2023
Debt Instrument [Line Items]    
Total Long-Term Debt $ 647.3 $ 517.2
Less: Unamortized Debt Issuance Costs 4.0 3.2
Long-Term Debt 643.3 514.0
Less: Current Portion [1] 4.9 4.9
Total Long-Term Debt, Less Current Portion 638.4 509.1
3.70% Senior Notes, Due August 1, 2026    
Debt Instrument [Line Items]    
Total Long-Term Debt 30.0 30.0
3.43% Senior Notes, Due December 18, 2029    
Debt Instrument [Line Items]    
Total Long-Term Debt 30.0 30.0
5.99% Senior Notes, Due August 21, 2034    
Debt Instrument [Line Items]    
Total Long-Term Debt 20.0  
Unitil Energy Systems Inc | First Mortgage Bonds 6.96% Senior Secured Notes, Due September 1, 2028    
Debt Instrument [Line Items]    
Total Long-Term Debt 8.0 10.0
Unitil Energy Systems Inc | First Mortgage Bonds 8.00% Senior Secured Notes, Due May 1, 2031    
Debt Instrument [Line Items]    
Total Long-Term Debt 10.5 12.0
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
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
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
Unitil Energy Systems Inc | First Mortgage Bonds 5.69% Senior Secured Notes Due August 21, 2054    
Debt Instrument [Line Items]    
Total Long-Term Debt 40.0  
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 6.0 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
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
Fitchburg Gas and Electric Light Company | 5.70 % Senior Note Due to July 2, 2033    
Debt Instrument [Line Items]    
Total Long-Term Debt 12.0 12.0
Fitchburg Gas and Electric Light Company | 5.54% Senior Notes Due to August 21, 2034    
Debt Instrument [Line Items]    
Total Long-Term Debt 12.5  
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
Fitchburg Gas and Electric Light Company | 5.99% Senior Notes, Due August 21, 2044    
Debt Instrument [Line Items]    
Total Long-Term Debt 12.5  
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
Fitchburg Gas and Electric Light Company | 5.96% Senior Note Due to July 2, 2053    
Debt Instrument [Line Items]    
Total Long-Term Debt 13.0 13.0
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 | 5.54% Senior Notes Due to August 21, 2034    
Debt Instrument [Line Items]    
Total Long-Term Debt 25.0  
Northern Utilities Inc | 3.78% Senior Notes, Due September 15, 2040    
Debt Instrument [Line Items]    
Total Long-Term Debt 40.0 40.0
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 | 7.72% Senior Notes, Due December 3, 2038    
Debt Instrument [Line Items]    
Total Long-Term Debt 50.0 50.0
Northern Utilities Inc | 5.74% Senior Notes, Due August 21, 2039    
Debt Instrument [Line Items]    
Total Long-Term Debt 15.0  
Northern Utilities Inc | 4.42% Senior Notes, Due October 15, 2044    
Debt Instrument [Line Items]    
Total Long-Term Debt 50.0 50.0
Northern Utilities Inc | 4.04% Senior Notes, Due September 12, 2049    
Debt Instrument [Line Items]    
Total Long-Term Debt 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
Granite State Gas Transmission Inc | 5.74% Senior Notes, Due August 21, 2034    
Debt Instrument [Line Items]    
Total Long-Term Debt 10.0  
Unitil Realty Corp Member | 2.64% Senior Notes, Due December 18, 2030    
Debt Instrument [Line Items]    
Total Long-Term Debt $ 3.8 $ 4.0
[1] The Current Portion of Long-Term Debt includes sinking fund payments.