Debt and Financing Arrangements - Additional Information (Detail) - USD ($)			9 Months Ended		12 Months Ended
	Jul. 24, 2015	Oct. 04, 2013	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Oct. 15, 2014	Apr. 30, 2014
Line of Credit Facility [Line Items]
Reclassification of short-term debt to long-term debt				$ 42,300,000
Proceeds from capital lease obligation			$ 11,400,000
Accounts Payable			21,200,000	19,300,000	$ 44,200,000
Guarantee outstanding			13,600,000
Northern Utilities Inc
Line of Credit Facility [Line Items]
Natural gas storage inventory			9,900,000	13,600,000	15,100,000
Accounts Payable			100,000	200,000	1,000,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility			120,000,000	120,000,000	$ 120,000,000
Proceeds from lines of credit			77,100,000	139,900,000
Repayments of lines of credit			$ 102,300,000	$ 156,400,000
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
Credit Facility | Capital Lease Obligations
Line of Credit Facility [Line Items]
Revolving credit facility							$ 15,000,000
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%
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%	8.00%
Long-term debt, maturity date			Aug. 01, 2017	Aug. 01, 2017	Aug. 01, 2017
Senior Secured Notes			$ 1,300,000	$ 1,900,000	$ 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%	7.15%
Long-term debt, maturity date			Dec. 15, 2018	Dec. 15, 2018	Dec. 15, 2018
Senior Notes			$ 10,000,000	$ 10,000,000	$ 10,000,000