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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(a))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=84165509&loc=d3e56426-112766

Debt and Financing Arrangements - Additional Information (Detail) - USD ($)			1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 18, 2020	Sep. 15, 2020	Jul. 25, 2018	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Line of Credit Facility [Line Items]
Issuance of long-term debt		$ 500,000
Weighted average interest rate on short term borrowings				1.20%	1.30%	1.20%	1.80%	1.70%
Capital lease obligation, current				$ 200,000		$ 200,000
Capital lease obligation, noncurrent				200,000		200,000
Accounts Payable				29,500,000	$ 25,100,000	29,500,000	$ 25,100,000	$ 33,200,000
Total rental expense under operating leases				500,000	500,000	1,400,000	1,400,000
Net Utility Plant				1,237,300,000	$ 1,185,300,000	1,237,300,000	1,185,300,000	1,193,200,000
Guarantee outstanding				700,000		700,000
Operating lease obligations						1,400,000	$ 1,400,000
Other current operating lease obligation				1,600,000		1,600,000
Other noncurrent operating lease obligation				$ 3,500,000		$ 3,500,000
Operating lease, weighted average remaining lease term				3 years 8 months 12 days	4 years	3 years 8 months 12 days	4 years
Operating lease, weighted average discount rate percentage				4.00%	4.40%	4.00%	4.40%
Long-term Debt, Gross				$ 515,100,000	$ 539,100,000	$ 515,100,000	$ 539,100,000	535,400,000
Payments of Debt Issuance Costs							500,000
Guarantee Term Period						1 year
Assets under Capital Leases [Member]
Line of Credit Facility [Line Items]
Net Utility Plant				700,000	1,100,000	$ 700,000	1,100,000	1,000,000.0
Net Utility Plant, accumulated amortization				$ 300,000	600,000	$ 300,000	600,000	500,000
Northern Utilities Inc
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization				65.00%		65.00%
Natural gas storage inventory				$ 9,700,000	6,000,000.0	$ 9,700,000	6,000,000.0	5,400,000
Accounts Payable				100,000	100,000	100,000	100,000	1,000,000.0
Unitil Reality Corp [Member]
Line of Credit Facility [Line Items]
Long-term debt, stated interest rate	2.64%
Long-term debt, maturity date	Dec. 18, 2020
Long-term Debt, Gross	$ 4,700,000
Payments of Debt Issuance Costs	$ 100,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility				$ 120,000,000.0	$ 120,000,000.0	120,000,000.0	$ 120,000,000.0	$ 120,000,000.0
Proceeds from lines of credit						170,700,000
Repayments of lines of credit						$ 194,900,000
Credit Facility \| 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 \| Second Amended Credit Facility \| London Interbank Offered Rate
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest			1.125%
Credit Facility \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Percentage of capitalization						The affirmative and negative covenants under the Credit Facility apply 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 Funded Debt to Capitalization (as each term is defined in the Credit Facility) cannot exceed 65%, tested on a quarterly basis. At September 30, 2021, September 30, 2020 and December 31, 2020, the Company was in compliance with the covenants contained in the Credit Facility in effect on those dates.
Three Point Five Eight Percentage Mortage BondsDue September Tweleve twenty Fourty [Member] \| Unitil Energy
Line of Credit Facility [Line Items]
Long-term debt, maturity date		Sep. 15, 2040
Three Point Five Eight Percentage Mortage BondsDue September Tweleve twenty Fourty [Member] \| Unitil Energy Systems Inc [Member]
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount		$ 27,500,000
Long-term debt, stated interest rate		3.58%
3.78% Senior Notes, Due September 15, 2040 \| Northern Utilities Inc
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount		$ 40,000,000
Long-term debt, stated interest rate		3.78%
Long-term debt, maturity date		Sep. 15, 2040
3.78% Senior Notes, Due September 15, 2040 \| Fitchburg Gas and Electric Light Company
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount		$ 27,500,000
Long-term debt, stated interest rate		3.78%
Long-term debt, maturity date		Sep. 15, 2040