Debt and Financing Arrangements - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 04, 2013	Apr. 30, 2014	Oct. 15, 2014
Line of Credit Facility [Line Items]
Long term debt repayments	$ 4,400,000	$ 500,000	$ 500,000
Weighted average interest rate on short term borrowings	1.60%	1.80%	2.00%
Standard & Poor's Ratings Services, Credit rating	BBB+
Proceeds from capital lease obligation	6,900,000
Accounts Payable	44,200,000	38,100,000
Total rental expense under operating leases	1,300,000	1,200,000	1,300,000
Net Utility Plant	733,700,000	665,600,000
Future minimum capital lease payments due current	471,000
Future minimum capital lease payments due noncurrent	7,500,000
Guarantee outstanding	39,100,000
Capital Lease Obligations
Line of Credit Facility [Line Items]
Revolving credit facility					15,000,000
Assets under Capital Leases
Line of Credit Facility [Line Items]
Net Utility Plant	9,700,000	1,400,000
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	15,100,000	12,500,000
Accounts Payable	1,000,000	2,700,000
Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization	65.00%
Master Lease Agreement
Line of Credit Facility [Line Items]
Future minimum capital lease payments due noncurrent	6,900,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility expiration date				Oct. 04, 2018
Revolving credit facility	120,000,000	120,000,000		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 September 30, 2014, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
Revolving Credit Facility | London Interbank Offered Rate
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest				1.375%
Bonds
Line of Credit Facility [Line Items]
Long term debt repayments	4,386,919	541,938	500,405
Debt repayment, 2015	4,035,633
Debt repayment, 2016	17,421,724
Debt repayment, 2017	17,160,985
Debt repayment, 2018	30,133,332
Debt repayment, 2019	18,800,000
Debt repayment, Thereafter	245,400,000
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%
Senior Secured Notes	1,700,000	2,300,000
8.00% Senior Secured Notes, Due August 1, 2017 | Unitil Realty Corp | Debt Outstanding, Principal Amount
Line of Credit Facility [Line Items]
Senior Secured Notes	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%
Senior Notes	10,000,000	10,000,000
7.15% Senior Notes, Due December 15, 2018 | Granite State Gas Transmission Inc | Debt Outstanding, Principal Amount
Line of Credit Facility [Line Items]
Senior Notes	$ 10,000,000