Debt and Financing Arrangements - Additional Information (Detail) - USD ($)		1 Months Ended	12 Months Ended
	Dec. 18, 2020	Jul. 25, 2018	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Sep. 15, 2020	Dec. 18, 2019	Sep. 12, 2019	Oct. 30, 2015
Line of Credit Facility [Line Items]
Issuance of long-term debt	$ 100,000					$ 500,000	$ 200,000	$ 200,000
Weighted average interest rate on short term borrowings			1.20%	1.70%	3.40%
Guarantee outstanding			$ 700,000
Capital lease obligation, current			100,000
Accounts Payable			52,400,000	$ 33,200,000
Total rental expense under operating leases			1,900,000	1,800,000	$ 1,400,000
Operating lease obligations			1,900,000	1,800,000
Net Utility Plant			1,257,200,000	$ 1,193,200,000
Other current operating lease obligation			$ 1,600,000
Operating lease, weighted average remaining lease term			3 years 6 months	3 years 9 months 18 days
Operating lease, weighted average discount rate percentage			3.90%	4.40%
Restriction on retained earnings for dividend payments			Therefore, there were no restrictions on the Company’s Retained Earnings at December 31, 2021 for the payment of dividends.
Amount available for the payment of dividends			$ 166,900,000
Retained Earnings			116,200,000	$ 103,700,000
Long term debt repayments			$ 25,800,000	$ 24,800,000	18,800,000
Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]			Other current operating lease obligation
Lease Obligations [Member]
Line of Credit Facility [Line Items]
Capital lease obligation, noncurrent			$ 200,000
Other noncurrent operating lease obligation			3,100,000
Unitil Energy, Fitchburg, Northern Utilities and Granite State
Line of Credit Facility [Line Items]
Amount available for the payment of dividends			$ 358,700,000
Credit Facility [Member] | Second Amended Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility		$ 120,000,000
Sublimit for the issuance of standby letters of credit		$ 25,000,000
Revolving credit facility termination date		Jul. 25, 2023
Increase in borrowing limit		$ 50,000,000
Credit Facility [Member] | London Interbank Offered Rate | Second Amended Credit Facility
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest		1.125%
3.43% Senior Notes, Due December 18, 2029
Line of Credit Facility [Line Items]
Long-term debt, stated interest rate			3.43%	3.43%
Long-term debt, maturity date			Dec. 18, 2029	Dec. 18, 2029
Bonds [Member]
Line of Credit Facility [Line Items]
Long term debt repayments			$ 25,800,000	$ 24,800,000	$ 18,800,000
Debt repayment, 2022			8,400,000
Debt repayment, 2023			6,900,000
Debt repayment, 2024			6,900,000
Debt repayment, 2025			5,000,000.0
Debt repayment, 2026			38,000,000.0
Debt repayment, Thereafter			444,400,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Revolving credit facility			120,000,000.0	120,000,000.0
Proceeds from lines of credit			239,100,000	248,900,000
Repayments of lines of credit			$ 229,700,000	252,800,000
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				1,000,000.0
Natural gas storage inventory			$ 8,300,000	$ 5,400,000
Unitil Service Corp
Line of Credit Facility [Line Items]
Capital lease obligation, total capitalized cost									$ 13,400,000
Northern Utilities Inc
Line of Credit Facility [Line Items]
Accounts Payable			$ 1,600,000
Long-term debt, aggregate principal amount						40,000,000
Long-term debt, stated interest rate								4.04%
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, aggregate principal amount								$ 40,000,000
Long-term debt, stated interest rate			4.04%	4.04%
Long-term debt, maturity date			Sep. 12, 2049	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
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						$ 27,500,000
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%
Unitil Corporation | 3.43% Senior Notes, Due December 18, 2029
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount							$ 30,000,000
Long-term debt, stated interest rate							3.43%
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						$ 27,500,000
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	$ 4,700,000
Long-term debt, stated interest rate	2.64%
Long-term debt, maturity date	Dec. 18, 2030
Assets under Capital Leases [Member]
Line of Credit Facility [Line Items]
Net Utility Plant			$ 700,000	$ 1,000,000.0
Net Utility Plant, accumulated amortization			$ 300,000	$ 500,000