Carrying value as of the balance sheet date of liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received that are used in an entity's business. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 852
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481372/852-10-55-10

DEBT AND FINANCING ARRANGEMENTS - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
	Sep. 29, 2022	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024	Jan. 29, 2025	Aug. 21, 2024
Line of Credit Facility [Line Items]
Weighted average interest rate on short term borrowings		5.60%	6.70%	6.50%
Repayments of debt				$ 1.0
Capital lease obligation, current		$ 0.1	$ 0.1	0.1
Capital lease obligation, noncurrent		0.3	0.3	0.4
Accounts Payable		42.9	37.9	49.7
Total rental expense under operating leases		0.6	0.5
Net Utility Plant		1,618.9	1,429.8	1,539.6
Operating lease obligations		0.6	0.5
Operating Lease Obligations		1.8	1.8	1.8
Operating Lease Obligations		$ 4.4	$ 3.8	4.9
Operating lease, weighted average remaining lease term		4 years	3 years 8 months 12 days
Operating lease, weighted average discount rate percentage		5.10%	4.60%
Assets under Capital Leases [Member]
Line of Credit Facility [Line Items]
Net Utility Plant		$ 0.6	$ 0.7	0.6
Net Utility Plant, accumulated amortization		$ 0.1	0.2	0.1
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	200.0
Proceeds from lines of credit		148.4
Repayments of lines of credit		81.3
Accounts Payable		0.6
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	$ 275.0
Credit Facility \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Natural gas storage inventory		$ 2.0
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%