Debt and Financing Arrangements - Additional Information (Detail) - USD ($) shares in Millions, $ in Millions				3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 06, 2023	Sep. 29, 2022	Sep. 22, 2022	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022	Mar. 31, 2023
Line of Credit Facility [Line Items]
Weighted average interest rate on short term borrowings				6.30%	2.10%	6.10%	1.70%	3.30%
Capital lease obligation, current				$ 0.1	$ 0.1	$ 0.1	$ 0.1	$ 0.1	$ 0.1
Capital lease obligation, noncurrent				0.3	0.1	0.3	0.1	0.1	0.3
Accounts Payable				36.7	34.3	36.7	34.3	68.6
Total rental expense under operating leases				0.5	0.5	1.0	0.9
Net Utility Plant				1,357.8	1,279.1	1,357.8	1,279.1	1,331.7
Operating lease obligations						1.0	0.9
Other current operating lease obligation				1.7	1.5	1.7	1.5	1.5	1.7
Other noncurrent operating lease obligation				$ 3.5	$ 2.9	$ 3.5	$ 2.9	2.8	$ 3.5
Operating lease, weighted average remaining lease term				3 years 8 months 12 days	3 years 3 months 18 days	3 years 8 months 12 days	3 years 3 months 18 days
Operating lease, weighted average discount rate percentage				4.50%	3.60%	4.50%	3.60%
Assets under Capital Leases [Member]
Line of Credit Facility [Line Items]
Net Utility Plant				$ 0.8	$ 0.7	$ 0.8	$ 0.7	0.6
Net Utility Plant, accumulated amortization				0.4	0.4	$ 0.4	0.4	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	$ 120.0	$ 200.0
Proceeds from lines of credit						193.6
Repayments of lines of credit						177.9
Accounts Payable				0.1		0.1
Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility				75.0		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]
Natural gas storage inventory				$ 8.7		8.7
Notes due date	Jul. 06, 2053
Cost of issue						$ 0.2
Issued note	13
Note issue percentage	5.96%
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 | Fitchburg [Member]
Line of Credit Facility [Line Items]
Notes due date	Jul. 06, 2033
Note issue percentage	5.70%
Note issued	12
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%