v3.25.2
DEBT AND FINANCING ARRANGEMENTS - Details On Long Term Debt (Detail) - USD ($)
$ in Millions
Jun. 30, 2025
Dec. 31, 2024
Jun. 30, 2024
Debt Instrument [Line Items]      
Total Long-Term Debt $ 644.5 $ 647.3 $ 514.4
Less: Unamortized Debt Issuance Costs 3.8 4.0 3.1
Long-Term Debt 640.7 643.3 511.3
Less: Current Portion 4.9 4.9 4.9
Total Long-Term Debt, Less Current Portion 635.8 638.4 506.4
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
5.99% Senior Notes, Due August 21, 2034      
Debt Instrument [Line Items]      
Total Long-Term Debt 20.0 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 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 9.0 10.5 10.5
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 | 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
Unitil Energy Systems Inc | 5.69% Senior Secured Notes, Due August 21, 2054      
Debt Instrument [Line Items]      
Total Long-Term Debt 40.0 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 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 4.8 6.0 6.0
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 | 5.70% Notes, Due July 2, 2033      
Debt Instrument [Line Items]      
Total Long-Term Debt 12.0 12.0 12.0
Fitchburg Gas and Electric Light Company | 5.54% Senior Notes, Due August 21, 2034      
Debt Instrument [Line Items]      
Total Long-Term Debt 12.5 12.5  
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 13.0
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 | 5.99% Senior Notes, Due August 21, 2044      
Debt Instrument [Line Items]      
Total Long-Term Debt 12.5 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 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 | 5.74% Senior Notes, Due August 21, 2039      
Debt Instrument [Line Items]      
Total Long-Term Debt 15.0 15.0  
Northern Utilities Inc | 5.54% Senior Notes, Due August 21, 2034      
Debt Instrument [Line Items]      
Total Long-Term Debt 25.0 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 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
Granite State Gas Transmission Inc | Senior Notes Five Point Seven Four Percent Due August Twenty One Twenty Thirty Four [Member]      
Debt Instrument [Line Items]      
Total Long-Term Debt 10.0 10.0  
Unitil Realty Corp [Member] | 2.64% Senior Secured Notes, Due December 18, 2030      
Debt Instrument [Line Items]      
Total Long-Term Debt $ 3.7 $ 3.8 $ 3.9