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).

Name:	us-gaap_AccountsPayableCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Debt and Financing Arrangements - Additional Information (Detail) - USD ($) $ in Millions				12 Months Ended
	Sep. 29, 2022	Sep. 22, 2022	Dec. 18, 2020	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Sep. 15, 2020
Line of Credit Facility [Line Items]
Issuance of long-term debt			$ 0.1				$ 0.5
Weighted average interest rate on short term borrowings				3.30%	1.20%	1.70%
Guarantee outstanding				$ 1.2
Total rental expense under operating leases				1.8	$ 1.9	$ 1.8
Accounts Payable				68.6	52.4
Operating lease obligations				1.8	1.9
Net Utility Plant				1,331.7	1,257.2
Other current operating lease obligation				1.5	1.6
Other noncurrent operating lease obligation				$ 2.8	$ 3.1
Operating lease, weighted average remaining lease term				3 years 4 months 24 days	3 years 6 months
Operating lease, weighted average discount rate percentage				3.90%	3.90%
Restriction on retained earnings for dividend payments				Therefore, there were no restrictions on the Company’s Retained Earnings at December 31, 2022 for the payment of dividends.
Amount available for the payment of dividends				$ 184.2
Retained Earnings				132.5	$ 116.2
Long term debt repayments				10.4	25.8	24.8
Finance Lease, Liability, Current				0.1	0.1
Finance Lease Liability Noncurrent				0.1	0.2
Subsidiaries [Member]
Line of Credit Facility [Line Items]
Amount available for the payment of dividends				$ 386.4
Credit Facility [Member] \| Third Amendment Credit Facility Member [Member]
Line of Credit Facility [Line Items]
Revolving credit facility	$ 200.0
Sublimit for the issuance of standby letters of credit	$ 25.0
3.43% Senior Notes, Due December 18, 2029
Line of Credit Facility [Line Items]
Long-term debt, stated interest rate				3.43%
Long-term debt, maturity date				Dec. 18, 2029
Bonds [Member]
Line of Credit Facility [Line Items]
Long term debt repayments				$ 10.4	25.8	$ 24.8
Debt repayment, 2023				6.9
Debt repayment, 2024				4.9
Debt repayment, 2025				4.9
Debt repayment, 2026				37.9
Debt repayment, 2027				55.7
Debt repayment, Thereafter				388.8
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Revolving credit facility				200.0	120.0
Proceeds from lines of credit				295.9	239.1
Repayments of lines of credit				$ 244.0	229.7
Revolving Credit Facility [Member] \| Credit Facility [Member]
Line of Credit Facility [Line Items]
Percentage of capitalization				The affirmative and negative covenants under the Credit Facility shall apply to Unitil until the Credit Facility terminates and all amounts borrowed under the 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
Accounts Payable					1.6
Natural gas storage inventory				$ 20.1	8.3
Revolving Credit Facility [Member] \| Credit Facility [Member] \| Third Amendment Credit Facility Member [Member]
Line of Credit Facility [Line Items]
Revolving credit facility termination date	Sep. 29, 2027
Northern Utilities Inc
Line of Credit Facility [Line Items]
Accounts Payable				$ 3.8
Long-term debt, aggregate principal amount							40.0
Total funded indebtedness as percentage of capitalization				65.00%
Northern Utilities Inc \| 4.04% Senior Notes, Due September 12, 2049
Line of Credit Facility [Line Items]
Long-term debt, stated interest rate				4.04%
Long-term debt, maturity date				Sep. 12, 2049
Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization				65.00%
Fitchburg Gas and Electric Light Company \| 3.78% Senior Notes, Due September 15, 2040
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount							$ 27.5
Long-term debt, stated interest rate							3.78%
Unitil Corporation \| Maximum [Member]
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization				70.00%
Northern Utilities And Fitchburg \| 3.78% Senior Notes, Due September 15, 2040
Line of Credit Facility [Line Items]
Long-term debt, stated interest rate							3.78%
Unitil Energy Systems Inc \| 3.58% Mortgage Bonds, Due September 12, 2040
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount							$ 27.5
Long-term debt, stated interest rate							3.58%
Unitil Reality Corp \| 2.64% Senior Secured Notes, Due December 18, 2030
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount			$ 4.7
Long-term debt, stated interest rate			2.64%
Long-term debt, maturity date			Dec. 18, 2030
Secured Overnight Financing Rate [Member] \| Revolving Credit Facility [Member] \| Credit Facility [Member] \| Third Amendment Credit Facility Member [Member]
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest	0.10%
Secured Overnight Financing Rate [Member] \| Revolving Credit Facility [Member] \| Credit Facility [Member] \| Third Amendment Credit Facility Member [Member] \| Maximum [Member]
Line of Credit Facility [Line Items]
Debt instrument variable interest rate additional spread		1.375%
Secured Overnight Financing Rate [Member] \| Revolving Credit Facility [Member] \| Credit Facility [Member] \| Third Amendment Credit Facility Member [Member] \| Minimum [Member]
Line of Credit Facility [Line Items]
Debt instrument variable interest rate additional spread		1.125%
Assets under Capital Leases [Member]
Line of Credit Facility [Line Items]
Net Utility Plant				$ 0.6	0.7
Net Utility Plant, accumulated amortization				$ 0.4	$ 0.3