DEBT AND FINANCING ARRANGEMENTS - Additional Information (Detail) - USD ($) shares in Millions, $ in Millions			3 Months Ended		12 Months Ended
	Jul. 06, 2023	Sep. 29, 2022	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Line of Credit Facility [Line Items]
Weighted average interest rate on short term borrowings			6.70%	5.90%	6.40%
Capital lease obligation, current			$ 0.1	$ 0.1	$ 0.1
Capital lease obligation, noncurrent			0.3	0.3	0.4
Accounts Payable			37.9	46.3	47.7
Total rental expense under operating leases			0.5	0.5
Net Utility Plant			1,429.8	1,341.3	1,420.9
Operating lease obligations			0.5	0.5
Other current operating lease obligation			1.8	1.8	1.9
Other noncurrent operating lease obligation			$ 3.8	$ 3.9	3.7
Operating lease, weighted average remaining lease term			3 years 8 months 12 days	3 years 9 months 18 days
Operating lease, weighted average discount rate percentage			4.60%	4.50%
Assets under Capital Leases [Member]
Line of Credit Facility [Line Items]
Net Utility Plant			$ 0.7	$ 0.9	0.7
Net Utility Plant, accumulated amortization			$ 0.2	0.4	0.2
Northern Utilities Inc
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization			65.00%
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility			$ 200.0	$ 200.0	$ 200.0
Proceeds from lines of credit			85.9
Repayments of lines of credit			78.0
Accounts Payable			$ 1.4
Debt Instrument, Covenant Description			The affirmative and negative covenants under the Credit Facility shall apply to Unitil until the Credit Facility terminates and all amounts borrowed under Credit Facility are paid in full (or, with respect to letters of credit, they are cash-collateralized). The only financial covenant in the Credit Facility provides that Unitil’s Funded Debt to Capitalization (as each term is defined in the Credit Facility) cannot exceed 65% tested on a quarterly basis
Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility		$ 75.0
Credit Facility | Third Amendment Credit Facility
Line of Credit Facility [Line Items]
Sublimit for the issuance of standby letters of credit		25.0
Revolving credit facility		$ 200.0
Credit Facility | Revolving Credit Facility
Line of Credit Facility [Line Items]
Subordinated Borrowing, Due Date	Jul. 02, 2053
cost of issue			$ 0.2
Debt Instrument, Interest Rate, Effective Percentage	5.96%
Debt Conversion, Converted Instrument, Warrants or Options Issued	13.0
Natural gas storage inventory			$ 3.3
Credit Facility | Revolving Credit Facility | Fitchburg [Member]
Line of Credit Facility [Line Items]
Debt Conversion, Converted Instrument, Shares Issued	12.0
Subordinated Borrowing, Due Date	Jul. 02, 2033
Debt Instrument, Interest Rate, Effective Percentage	5.70%
Credit Facility | Revolving Credit Facility | Third Amendment Credit Facility | Secured Overnight Financing Rate [Member]
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest		0.10%
Credit Facility | Revolving Credit Facility | Third Amendment Credit Facility | Secured Overnight Financing Rate [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Debt instrument variable interest rate additional spread		1.375%
Credit Facility | Revolving Credit Facility | Third Amendment Credit Facility | Secured Overnight Financing Rate [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Debt instrument variable interest rate additional spread		1.125%