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 ($)				1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 15, 2020	Dec. 18, 2019	Sep. 12, 2019	Jul. 25, 2018	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Oct. 30, 2015
Line of Credit Facility [Line Items]
Weighted average interest rate on short term borrowings					1.30%	3.40%		1.30%	3.40%	3.40%
Gain on divestiture of business, net							$ 9,800,000
Capital lease obligation, total capitalized cost					$ 485,000	$ 600,000		$ 485,000	$ 600,000	$ 500,000
Capital lease obligation, current					200,000	300,000		200,000	300,000	200,000
Capital lease obligation, noncurrent					300,000	300,000		300,000	300,000	300,000
Accounts Payable					25,100,000	22,900,000		25,100,000	22,900,000	37,600,000
Guarantee outstanding					6,200,000			6,200,000
Total rental expense under operating leases					500,000	400,000		1,400,000	1,100,000
Operating lease obligations								1,400,000
Net Utility Plant					1,185,300,000	1,083,900,000		1,185,300,000	1,083,900,000	1,111,500,000
Other current operating lease obligation					1,600,000	1,200,000		1,600,000	1,200,000	1,200,000
Other noncurrent operating lease obligation					$ 4,000,000.0	$ 3,000,000.0		$ 4,000,000.0	$ 3,000,000.0	$ 2,800,000
Issuance of long-term debt	$ 500,000	$ 200,000	$ 200,000
Operating lease, weighted average remaining lease term					4 years			4 years
Operating lease, weighted average discount rate percentage					4.40%			4.40%
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%		3.43%	3.43%	3.43%
Long-term debt, maturity date								Dec. 18, 2029	Dec. 18, 2029	Dec. 18, 2029
4.04% Senior Notes, Due September 12, 2049
Line of Credit Facility [Line Items]
Long-term debt, stated interest rate					4.04%	4.04%		4.04%	4.04%	4.04%
Long-term debt, maturity date								Sep. 12, 2049	Sep. 12, 2049	Sep. 12, 2049
Revolving Credit Facility [Member]
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								186,000,000.0
Repayments of lines of credit								$ 227,300,000
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 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, 2020, September 30, 2019 and December 31, 2019, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
Unitil Service Corp [Member]
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					100,000	100,000		$ 100,000	100,000	1,000,000.0
Natural gas storage inventory					6,000,000.0	$ 8,100,000		$ 6,000,000.0	$ 8,100,000	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%			4.04%
Long-term debt, maturity date			Sep. 12, 2049						Sep. 12, 2049
Northern Utilities Inc \| 3.78% Senior Notes, Due September 12, 2040
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount	$ 40,000,000
Long-term debt, maturity date	Sep. 15, 2040
Unitil Energy [Member] \| 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%
Long-term debt, maturity date	Sep. 15, 2040
Fitchburg Gas and Electric Light Company \| 3.78% Senior Notes, 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	3.78%
Long-term debt, maturity date	Sep. 15, 2040
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 12, 2040
Line of Credit Facility [Line Items]
Long-term debt, stated interest rate	3.78%
Assets under Capital Leases [Member]
Line of Credit Facility [Line Items]
Net Utility Plant					1,100,000	$ 1,300,000		$ 1,100,000	$ 1,300,000	1,200,000
Net Utility Plant, accumulated amortization					$ 600,000	$ 600,000		$ 600,000	$ 600,000	$ 600,000