Debt and Financing Arrangements - Additional Information (Detail) - USD ($)				1 Months Ended	12 Months Ended
	Dec. 18, 2020	Sep. 15, 2020	Sep. 12, 2019	Jul. 25, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 18, 2019	Oct. 30, 2015
Line of Credit Facility [Line Items]
Weighted average interest rate on short term borrowings					1.70%	3.40%	3.30%
Accounts Payable					$ 33,200,000	$ 37,600,000
Guarantee outstanding					1,300,000
Total rental expense under operating leases					1,800,000	1,400,000	$ 2,200,000
Operating lease obligations					1,800,000	1,400,000
Net Utility Plant					$ 1,193,200,000	$ 1,111,500,000
Issuance of long-term debt	$ 100,000	$ 500,000	$ 200,000					$ 200,000
Operating lease, weighted average remaining lease term					3 years 9 months 18 days	3 years 10 months 24 days
Operating lease, weighted average discount rate percentage					4.40%	5.20%
Restriction on retained earnings for dividend payments					Therefore, there were no restrictions on the Company’s Retained Earnings at December 31, 2020 for the payment of dividends.
Amount available for the payment of dividends					$ 133,800,000
Retained Earnings					103,700,000	$ 94,100,000
Long term debt repayments					24,800,000	18,800,000	30,100,000
Lease Obligations [Member]
Line of Credit Facility [Line Items]
Capital lease obligation, total capitalized cost					424,000	500,000
Capital lease obligation, current					200,000	200,000
Capital lease obligation, noncurrent					200,000	300,000
Other current operating lease obligation					1,500,000	1,200,000
Other noncurrent operating lease obligation					3,700,000	$ 2,800,000
Unitil Energy, Fitchburg, Northern Utilities and Granite State
Line of Credit Facility [Line Items]
Amount available for the payment of dividends					$ 325,800,000
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
4.04% Senior Notes, Due September 12, 2049
Line of Credit Facility [Line Items]
Long-term debt, maturity date						Sep. 12, 2049
Bonds [Member]
Line of Credit Facility [Line Items]
Long term debt repayments					$ 24,800,000	$ 18,800,000	$ 30,100,000
Debt repayment, 2021					8,800,000
Debt repayment, 2022					23,400,000
Debt repayment, 2023					6,900,000
Debt repayment, 2024					7,000,000.0
Debt repayment, 2025					7,000,000.0
Debt repayment, Thereafter					482,300,000
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					248,900,000	252,700,000
Repayments of lines of credit					$ 252,800,000	276,900,000
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. At December 31, 2020 and December 31, 2019, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
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. At December 31, 2020 and December 31, 2019, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
Accounts Payable						1,000,000.0
Natural gas storage inventory					$ 5,400,000	6,500,000
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					1,000,000.0	1,000,000.0
Long-term debt, aggregate principal amount		$ 40,000,000
Long-term debt, stated interest rate			4.04%
Natural gas storage inventory					$ 5,400,000	$ 6,500,000
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%
Long-term debt, maturity date	Dec. 18, 2029
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%
Long-term debt, maturity date		Sep. 15, 2040
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					$ 1,000,000.0	$ 1,200,000
Net Utility Plant, accumulated amortization					$ 500,000	$ 600,000