Debt and Financing Arrangements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified							1 Months Ended	3 Months Ended			1 Months Ended									1 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Mar. 31, 2014 Northern Utilities Inc	Dec. 31, 2013 Northern Utilities Inc	Mar. 31, 2013 Northern Utilities Inc	Oct. 04, 2013 Revolving Credit Facility	Mar. 31, 2014 Revolving Credit Facility	Dec. 31, 2013 Revolving Credit Facility	Mar. 31, 2013 Revolving Credit Facility	Oct. 04, 2013 Revolving Credit Facility London Interbank Offered Rate	Mar. 31, 2014 8.00% Senior Secured Notes, Due Through August 1, 2017 Unitil Realty Corp	Dec. 31, 2013 8.00% Senior Secured Notes, Due Through August 1, 2017 Unitil Realty Corp	Mar. 31, 2013 8.00% Senior Secured Notes, Due Through August 1, 2017 Unitil Realty Corp	Mar. 31, 2014 8.00% Senior Secured Notes, Due Through August 1, 2017 Unitil Realty Corp Debt Outstanding, Principal Amount	Mar. 31, 2014 7.15% Senior Notes, Due December 15, 2018 Granite State Gas Transmission Inc	Dec. 31, 2013 7.15% Senior Notes, Due December 15, 2018 Granite State Gas Transmission Inc	Mar. 31, 2013 7.15% Senior Notes, Due December 15, 2018 Granite State Gas Transmission Inc	Mar. 31, 2014 7.15% Senior Notes, Due December 15, 2018 Granite State Gas Transmission Inc Debt Outstanding, Principal Amount	Apr. 30, 2014 Construction line of credit Subsequent Event
Line of Credit Facility [Line Items]
Estimated Fair Value of Long-Term Debt	$ 332.8
Revolving credit facility expiration date							Oct. 04, 2018													Aug. 31, 2015
Line of credit facility, maximum borowing capacity							120.0	120.0	120.0	60.0										15.0
Sublimit for the issuance of standby letters of credit							25
Credit facility, daily fluctuating rate of interest											1.375%
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, 2014, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
Capital lease term in years																				5 years
Line of credit facility, funding received																				4.4
Outstanding inventory				3.2	12.5	3.3
Accounts Payable	31.8	38.1	27.1	2.2	2.7	1.7
Guarantee outstanding	31.0
Guarantee end date	Apr. 30, 2015
Senior Secured Notes												2.2	2.3	2.7	2.2
Senior Notes																$ 10.0	$ 10.0	$ 10.0	$ 10.0