Debt and Financing Arrangements - Additional Information (Detail) - USD ($)			1 Months Ended	12 Months Ended
	Jul. 25, 2018	Nov. 01, 2017	Nov. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Oct. 30, 2015
Line of Credit Facility [Line Items]
Amount available for the payment of dividends				$ 137,400,000
Retained Earnings				$ 72,000,000	$ 60,800,000
Restriction on retained earnings for dividend payments				Therefore, there were no restrictions on the Company’s Retained Earnings at December 31, 2018 for the payment of dividends.
Issuance of long-term debt		$ 700,000	$ 500,000
Long term debt repayments				$ 30,100,000	$ 17,200,000	$ 19,000,000
Weighted average interest rate on short term borrowings				3.30%	2.40%	1.80%
Capital lease obligation, current				$ 3,100,000	$ 3,100,000
Capital lease obligation, noncurrent				2,700,000	5,700,000
Accounts Payable				42,600,000	41,500,000
Total rental expense under operating leases				2,200,000	2,000,000	$ 1,800,000
Net Utility Plant				1,036,800,000	971,500,000
Guarantee outstanding				4,300,000
Financing Arrangements
Line of Credit Facility [Line Items]
Capital lease obligation, current				2,800,000
Capital lease obligation, noncurrent				2,300,000
Assets under Capital Leases
Line of Credit Facility [Line Items]
Net Utility Plant				15,000,000	15,000,000
Net Utility Plant, accumulated amortization				$ 1,700,000	700,000
Northern Utilities Inc
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization				65.00%
Natural gas storage inventory				$ 8,400,000	8,500,000
Accounts Payable				$ 900,000	3,100,000
Fitchburg Gas and Electric Light Company
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization				65.00%
Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization				65.00%
Unitil Corporation | Maximum
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization				70.00%
Unitil Service Corp.
Line of Credit Facility [Line Items]
Capital lease obligation, total capitalized cost							$ 13,400,000
Capital lease obligation, maturity period				Sep. 30, 2020
Capital lease obligation, current				$ 2,800,000
Capital lease obligation, noncurrent				2,300,000
Unitil Energy, Fitchburg, Northern Utilities and Granite State
Line of Credit Facility [Line Items]
Amount available for the payment of dividends				249,200,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility				120,000,000	120,000,000
Proceeds from lines of credit				265,600,000	234,900,000
Repayments of lines of credit				$ 221,100,000	$ 278,500,000
3.52% Senior Notes, Due November 1, 2027 | Northern Utilities Inc
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount		$ 20,000,000
Long-term debt, stated interest rate		3.52%		3.52%	3.52%
Long-term debt, maturity date		Nov. 01, 2027		Nov. 01, 2027	Nov. 01, 2027
3.52% Senior Notes, Due November 1, 2027 | Fitchburg Gas and Electric Light Company
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount		$ 10,000,000
Long-term debt, stated interest rate		3.52%		3.52%	3.52%
Long-term debt, maturity date		Nov. 01, 2027		Nov. 01, 2027	Nov. 01, 2027
4.32% Senior Notes, Due November 1, 2047 | Northern Utilities Inc
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount		$ 30,000,000
Long-term debt, stated interest rate		4.32%		4.32%	4.32%
Long-term debt, maturity date		Nov. 01, 2047		Nov. 01, 2047	Nov. 01, 2047
4.32% Senior Notes, Due November 1, 2047 | Fitchburg Gas and Electric Light Company
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount		$ 15,000,000
Long-term debt, stated interest rate		4.32%		4.32%	4.32%
Long-term debt, maturity date		Nov. 01, 2047		Nov. 01, 2047	Nov. 01, 2047
3.72% Senior Notes, Due November 1, 2027 | Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount		$ 15,000,000
Long-term debt, stated interest rate		3.72%		3.72%	3.72%
Long-term debt, maturity date		Nov. 01, 2027		Nov. 01, 2027	Nov. 01, 2027
Bonds
Line of Credit Facility [Line Items]
Long term debt repayments				$ 30,100,000	$ 17,200,000	$ 19,000,000
Debt repayment, 2019				18,800,000
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, Thereafter				$ 327,200,000
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, 2018 and December 31, 2017, 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 | 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