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	12 Months Ended
	Dec. 18, 2019	Sep. 12, 2019	Nov. 30, 2018	Jul. 25, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Oct. 30, 2015
Line of Credit Facility [Line Items]
Amount available for the payment of dividends					$ 123,100,000
Retained Earnings					$ 94,100,000	$ 72,000,000.0
Restriction on retained earnings for dividend payments					Therefore, there were no restrictions on the Company’s Retained Earnings at December 31, 2019 for the payment of dividends.
Issuance of long-term debt	$ 200,000	$ 200,000
Long term debt repayments					$ 18,800,000	$ 30,100,000	$ 17,200,000
Weighted average interest rate on short term borrowings					3.40%	3.30%	2.40%
Capital lease obligation, total capitalized cost					$ 546,000	$ 5,800,000
Capital lease obligation, current					200,000	3,100,000
Capital lease obligation, noncurrent	300,000				300,000	2,700,000
Accounts Payable					37,600,000	42,600,000
Total rental expense under operating leases					1,400,000	2,200,000	$ 2,000,000.0
Net Utility Plant					1,111,500,000	1,036,800,000
Guarantee outstanding					6,200,000
Operating lease obligations					1,400,000
Other current operating lease obligation					1,200,000	1,200,000
Other noncurrent operating lease obligation					$ 2,800,000
Operating lease, weighted average remaining lease term					3 years 10 months 24 days
Operating lease, weighted average discount rate percentage					5.20%
Assets under Capital Leases [Member]
Line of Credit Facility [Line Items]
Net Utility Plant					$ 1,200,000	15,000,000.0
Net Utility Plant, accumulated amortization					$ 600,000	1,700,000
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]
Total funded indebtedness as percentage of capitalization					65.00%
Natural gas storage inventory					$ 6,500,000	8,400,000
Accounts Payable						900,000
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%
Unitil Corporation \| Maximum [Member]
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization					70.00%
Unitil Energy, Fitchburg, Northern Utilities and Granite State
Line of Credit Facility [Line Items]
Amount available for the payment of dividends					$ 308,400,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility					120,000,000.0	120,000,000.0
Proceeds from lines of credit					252,700,000	265,600,000
Repayments of lines of credit					$ 276,900,000	$ 221,100,000
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 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, 2019 and December 31, 2018, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
4.18% First Mortgage Bonds, Due November 30, 2048
Line of Credit Facility [Line Items]
Issuance of long-term debt			$ 500,000
4.18% First Mortgage Bonds, Due November 30, 2048 \| Unitil Energy
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount			$ 30,000,000
Long-term debt, stated interest rate			4.18%
Long-term debt, maturity date			Nov. 30, 2048
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
3.43% Senior Notes, Due December 18, 2029 \| Unitil Corporation
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
4.04% Senior Notes, Due September 12, 2049 \| Northern Utilities Inc
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	Sep. 12, 2049
Bonds [Member]
Line of Credit Facility [Line Items]
Long term debt repayments						$ 30,100,000	$ 17,200,000
Debt repayment, 2020					$ 19,800,000
Debt repayment, 2021					8,600,000
Debt repayment, 2022					28,200,000
Debt repayment, 2023					6,700,000
Debt repayment, 2024					6,700,000
Debt repayment, Thereafter					$ 390,500,000