Debt and Financing Arrangements - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
	Sep. 29, 2022	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Line of Credit Facility [Line Items]
Weighted average interest rate on short term borrowings		5.90%	1.35%	3.26%
Capital lease obligation, current		$ 0.1	$ 0.1	$ 0.1
Capital lease obligation, noncurrent		0.3	0.2	0.1
Accounts Payable		46.3	39.0	68.6
Total rental expense under operating leases		0.5	0.5
Net Utility Plant		1,341.3	1,259.9	1,331.7
Guarantee outstanding		1.0
Operating lease obligations		0.5	0.5
Other current operating lease obligation		1.8	1.6	1.5
Other noncurrent operating lease obligation		$ 3.9	$ 3.2	2.8
Operating lease, weighted average remaining lease term		3 years 9 months 18 days	3 years 6 months
Operating lease, weighted average discount rate percentage		4.50%	3.70%
Assets under Capital Leases [Member]
Line of Credit Facility [Line Items]
Net Utility Plant		$ 0.9	$ 0.6	0.6
Net Utility Plant, accumulated amortization		$ 0.4	0.3	0.4
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	120.0	$ 200.0
Proceeds from lines of credit		135.6	135.6
Repayments of lines of credit		111.4	$ 111.4
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]
Natural gas storage inventory		4.0
Accounts Payable		$ 3.9
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 | Revolving Credit Facility | Third Amendment Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility expiration date	Sep. 29, 2027
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%