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Details on Long Term Debt (Detail) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021	Sep. 30, 2021
Debt Instrument [Line Items]
Total Long-Term Debt	$ 504.7	$ 509.6	$ 515.1
Less: Unamortized Debt Issuance Costs	3.4	3.6	3.7
Long-Term Debt	501.3	506.0	511.4
Less: Current Portion	8.2	8.2	10.1
Total Long-Term Debt, Less Current Portion	493.1	497.8	501.3
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 8.49% Senior Secured Notes, Due October 14, 2024
Debt Instrument [Line Items]
Total Long-Term Debt	1.5	1.5	3.0
Unitil Energy Systems Inc | First Mortgage Bonds 6.96% Senior Secured Notes, Due September 1, 2028
Debt Instrument [Line Items]
Total Long-Term Debt	12.0	14.0	14.0
Unitil Energy Systems Inc | First Mortgage Bonds 8.00% Senior Secured Notes, Due May 1, 2031
Debt Instrument [Line Items]
Total Long-Term Debt	13.5	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	30.0
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
Fitchburg Gas and Electric Light Company | 6.79% Senior Notes, Due October 15, 2025
Debt Instrument [Line Items]
Total Long-Term Debt	6.0	6.0	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	8.4	9.6	9.6
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
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
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 | 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
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
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
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	$ 4.3	$ 4.5	$ 4.5

X

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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.

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