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
	Jul. 24, 2015	Oct. 04, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Oct. 30, 2015	Oct. 15, 2014
Line of Credit Facility [Line Items]
Amount available for the payment of dividends			$ 194,600,000
Retained earnings			$ 45,100,000	$ 38,400,000
Restriction on retained earnings for dividend payments			There were no restrictions on the Company's Retained Earnings at December 31, 2015 for the payment of dividends
Long term debt repayments			$ 7,400,000	$ 4,400,000	$ 500,000
Weighted average interest rate on short term borrowings			1.50%	1.60%	1.80%
Capital lease obligation, total capitalized cost						$ 13,400,000
Capital lease obligation, maturity period			Sep. 30, 2020
Accounts Payable			$ 33,300,000	$ 44,200,000
Total rental expense under operating leases			1,700,000	1,300,000	$ 1,200,000
Net Utility Plant			808,900,000	733,700,000
Capital lease obligations, current			3,100,000	500,000
Capital lease obligations, Noncurrent			11,000,000	7,500,000
Guarantee outstanding			19,300,000
Assets under Capital Leases
Line of Credit Facility [Line Items]
Net Utility Plant			15,300,000	9,700,000
Net Utility Plant, accumulated amortization			$ 800,000	800,000
Unitil Corporation \| Maximum
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization			70.00%
Fitchburg Gas and Electric Light Company
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			$ 10,800,000	15,100,000
Accounts Payable			$ 600,000	1,000,000
Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization			65.00%
Unitil Energy, Fitchburg, Northern Utilities and Granite State
Line of Credit Facility [Line Items]
Amount available for the payment of dividends			$ 163,100,000
Financing Arrangements
Line of Credit Facility [Line Items]
Capital lease obligations, current			2,600,000
Capital lease obligations, Noncurrent			10,400,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility			120,000,000	120,000,000
Proceeds from lines of credit			140,300,000	179,400,000
Repayments of lines of credit			$ 127,600,000	$ 210,300,000
4.42% Senior Unsecured Notes due October 15, 2044 \| Northern Utilities Inc
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount							$ 50,000,000
Long-term debt, stated interest rate							4.42%
Long-term debt, maturity date			Oct. 15, 2044
8.00% Senior Secured Notes, Due August 1, 2017 \| Unitil Realty Corp
Line of Credit Facility [Line Items]
Long-term debt, stated interest rate			8.00%	8.00%
Long-term debt, maturity date			Aug. 01, 2017	Aug. 01, 2017
Senior Secured Notes			$ 1,100,000	$ 1,700,000
7.15% Senior Notes, Due December 15, 2018 \| Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Long-term debt, stated interest rate			7.15%	7.15%
Long-term debt, maturity date			Dec. 15, 2018	Dec. 15, 2018
Senior Notes			$ 10,000,000	$ 10,000,000
Bonds
Line of Credit Facility [Line Items]
Long term debt repayments			7,400,000	$ 4,400,000	$ 500,000
Debt repayment, 2016			17,400,000
Debt repayment, 2017			17,200,000
Debt repayment, 2018			30,100,000
Debt repayment, 2019			18,800,000
Debt repayment, 2020			19,800,000
Debt repayment, Thereafter			$ 222,200,000
Credit Facility \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility expiration date		Oct. 04, 2018
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
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 June 30, 2015, June 30, 2014 and December 31, 2014, 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
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest		1.375%
Credit Facility \| Amended Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility expiration date	Oct. 04, 2020
Credit Facility \| Amended Credit Facility \| London Interbank Offered Rate
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest	1.25%