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 ($)			12 Months Ended
	Nov. 01, 2017	Oct. 04, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 01, 2016	Oct. 30, 2015
Line of Credit Facility [Line Items]
Amount available for the payment of dividends			$ 125,000,000
Retained Earnings			$ 60,800,000	$ 52,200,000
Restriction on retained earnings for dividend payments			Therefore, there were no restrictions on the Company's Retained Earnings at December 31, 2017 for the payment of dividends.
Issuance of long-term debt	$ 700,000		$ 300,000
Long term debt repayments			$ 17,200,000	$ 19,000,000	$ 7,400,000
Weighted average interest rate on short term borrowings			2.40%	1.80%	1.50%
Capital lease obligation, current			$ 3,100,000	$ 3,000,000
Capital lease obligation, noncurrent			5,700,000	8,300,000
Accounts Payable			41,500,000	32,400,000
Total rental expense under operating leases			2,000,000	1,800,000	$ 1,700,000
Net Utility Plant			971,500,000	883,400,000
Guarantee outstanding			17,900,000
Financing Arrangements
Line of Credit Facility [Line Items]
Capital lease obligation, current			2,700,000
Capital lease obligation, noncurrent			5,100,000
Assets under Capital Leases
Line of Credit Facility [Line Items]
Net Utility Plant			15,000,000	15,300,000
Net Utility Plant, accumulated amortization			$ 700,000	1,000,000
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			$ 2,700,000
Capital lease obligation, noncurrent			$ 5,100,000
Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization			65.00%
Northern Utilities Inc
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization			65.00%
Natural gas storage inventory			$ 8,500,000	9,900,000
Accounts Payable			$ 3,100,000	2,100,000
Fitchburg Gas and Electric Light Company
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 Energy, Fitchburg, Northern Utilities and Granite State
Line of Credit Facility [Line Items]
Amount available for the payment of dividends			$ 238,300,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility			120,000,000	120,000,000
Proceeds from lines of credit			234,900,000	218,200,000
Repayments of lines of credit			$ 278,500,000	$ 178,300,000
Credit Facility \| Revolving 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
Credit Facility by an aggregate additional amount		$ 30,000,000
Revolving credit facility termination date		Oct. 04, 2020
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, 2017 and December 31, 2016, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
Credit Facility \| Revolving Credit Facility \| London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest		1.25%
7.15% Senior Notes, Due December 15, 2018 \| Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Long-term debt, maturity date			Dec. 15, 2018	Dec. 15, 2018
Long-term debt, stated interest rate			7.15%	7.15%
Senior Notes			$ 3,300,000
3.70% Senior Notes, Due August 1, 2026
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount						$ 30,000,000
Long-term debt, maturity date			Aug. 01, 2026	Aug. 01, 2026
Long-term debt, stated interest rate			3.70%	3.70%		3.70%
Bonds
Line of Credit Facility [Line Items]
Long term debt repayments			$ 17,200,000	$ 19,000,000	$ 7,400,000
Debt repayment, 2018			30,100,000
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, Thereafter			$ 303,900,000
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, maturity date	Nov. 01, 2027		Nov. 01, 2027	Nov. 01, 2027
Long-term debt, stated interest rate	3.52%		3.52%	3.52%
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, maturity date	Nov. 01, 2027		Nov. 01, 2027	Nov. 01, 2027
Long-term debt, stated interest rate	3.52%		3.52%	3.52%
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, maturity date	Nov. 01, 2047		Nov. 01, 2047	Nov. 01, 2047
Long-term debt, stated interest rate	4.32%		4.32%	4.32%
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, maturity date	Nov. 01, 2047		Nov. 01, 2047	Nov. 01, 2047
Long-term debt, stated interest rate	4.32%		4.32%	4.32%
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, maturity date	Nov. 01, 2027		Nov. 01, 2027	Nov. 01, 2027
Long-term debt, stated interest rate	3.72%		3.72%	3.72%