Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(8)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(14))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482099/360-10-50-1

Debt And Financing Arrangements - Additional Information (Detail) - USD ($) $ in Millions			3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Oct. 31, 2025	Sep. 29, 2022	Sep. 30, 2025	Sep. 30, 2024	Jun. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024	Jul. 08, 2025	Jan. 29, 2025	Aug. 21, 2024
Line of Credit Facility [Line Items]
Weighted average interest rate on short term borrowings			5.50%	6.60%		5.50%	6.60%	6.50%
Repayments of debt				$ 1.0			$ 1.0
Capital lease obligation, current			$ 0.2	0.1		$ 0.2	0.1	$ 0.1
Capital lease obligation, noncurrent			0.3	0.3		0.3	0.3	0.4
Total rental expense under operating leases			0.6	0.5	$ 1.6	1.7
Debt instrument maturity date	Oct. 31, 2026
Net Utility Plant			1,682.9	1,502.4		1,682.9	1,502.4	1,539.6
Operating lease obligations						1.7	1.6
Operating Lease Obligations			1.9	1.6		1.9	1.6	1.8
Operating Lease Obligations			$ 4.9	$ 3.0		$ 4.9	$ 3.0	4.9
Operating lease, weighted average remaining lease term			3 years 10 months 24 days	3 years 6 months		3 years 10 months 24 days	3 years 6 months
Operating lease, weighted average discount rate percentage			5.30%	4.60%		5.30%	4.60%
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			0.3	0.3		0.3	0.3	0.1
Base Rate [Member]
Line of Credit Facility [Line Items]
Debt instrument variable interest rate additional spread	0.25%
Federal Funds Rate
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest	0.50%
SOFR
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest	0.10%
Debt instrument variable interest rate additional spread	1.25%
Bangor Natural Gas [Member]
Line of Credit Facility [Line Items]
Net Utility Plant			66.7			$ 66.7
Northern Utilities Inc [Member]
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			275.0	$ 200.0		$ 275.0	$ 200.0	200.0
Proceeds from lines of credit						380.0
Repayments of lines of credit						368.3
Senior unsecured delayed draw term loan facility
Line of Credit Facility [Line Items]
Acquisition amount	$ 86.0
Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility		$ 75.0
Description of variable rate basis	The facility provides that the Company has an option for determining whether interest on loans under the facility will bear interest based on a Base Rate plus an applicable margin of 0.25% or based on a one month Term SOFR plus a SOFR adjustment of 0.10% plus an applicable margin of 1.25%. The Base Rate is equal to the highest of the (a) Federal Funds Rate plus 0.50%, (b) the rate of interest in effect for such day as publicly announced from time to time by The Bank of Nova Scotia as its "prime rate", or (c) one month Term SOFR plus a SOFR adjustment of 0.10% plus 1.00%.
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		$ 275.0
Credit Facility \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Cost of issue						0.2
Natural gas storage inventory			$ 14.4			$ 14.4
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 \| Revolving Credit Facility \| Third Amendment Credit Facility
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest		0.10%
Credit Facility \| Revolving Credit Facility \| Third Amendment Credit Facility \| 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 \| Minimum [Member]
Line of Credit Facility [Line Items]
Debt instrument variable interest rate additional spread		1.125%
Senior Notes Five Point Nine Nine Percent Due Twenty Thirty Four [Member] \| Fitchburg
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount											$ 12.5
Long-term debt, stated interest rate											5.54%
Senior Notes Five Point Nine Nine Percent Due Twenty Thirty Four [Member] \| Unitil Corporation [Member]
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount											$ 20.0
Long-term debt, stated interest rate											5.99%
Senior Notes Five Point Nine Nine Percent Due Twenty Thirty Four [Member] \| Northern Utilities Inc [Member]
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount											$ 25.0
Long-term debt, stated interest rate											5.54%
Senior Notes Five Point Nine Nine Percent Due Twenty Thirty Four [Member] \| Granite State Gas Transmission Inc [Member]
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount											$ 10.0
Debt Instrument, Interest Rate, Effective Percentage											5.74%
Senior Notes Five Point Nine Nine Percent Due Twenty Forty Four [Member] \| Fitchburg
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount											$ 12.5
Long-term debt, stated interest rate											5.99%
Senior Notes Five Point Six Nine Percent Due Twenty Fifty Four [Member] \| Unitil Energy Systems Inc
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount											$ 40.0
Long-term debt, stated interest rate											5.69%
Senior Notes Five Point Seven Four Percent Due Twenty Thirty Nine [Member] \| Northern Utilities Inc [Member]
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount											$ 15.0
Long-term debt, stated interest rate											5.74%
Senior Notes Five Point Seven Zero Percent Due Twenty Thirty [Member] \| Bangor Natural Gas [Member]
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount									$ 14.0
Long-term debt, stated interest rate									5.70%
Senior Notes Six Point Three One Percent Due Twenty Thirty Five [Member] \| Bangor Natural Gas [Member]
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount									$ 18.0
Long-term debt, stated interest rate									6.31%