Debt and Financing Arrangements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	0 Months Ended
	Mar. 31, 2015	Oct. 04, 2013	Dec. 31, 2014	Mar. 31, 2014	Apr. 30, 2014	Oct. 15, 2014
Line of Credit Facility [Line Items]
Proceeds from capital lease obligation	$ 7.7
Accounts Payable	34.8		44.2	31.8
Guarantee outstanding	37.1
Capital Lease Obligations
Line of Credit Facility [Line Items]
Revolving credit facility					15.0
Northern Utilities Inc
Line of Credit Facility [Line Items]
Natural gas storage inventory	5.6		15.1	3.2
Accounts Payable	2.0		1.0	2.2
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility expiration date		Oct. 04, 2018
Revolving credit facility	120.0	120.0	120.0	120.0
Sublimit for the issuance of standby letters of credit		25
Credit Facility by an aggregate additional amount		30
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 March 31, 2015, March 31, 2014 and December 31, 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%
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
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%	8.00%
Senior Secured Notes	1.5		1.7	2.2
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	1.5
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%
Senior Notes	10.0		10.0	10.0
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	$ 10.0