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Debt and Financing Arrangements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended							0 Months Ended	6 Months Ended				0 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Jun. 30, 2014 Northern Utilities Inc	Dec. 31, 2013 Northern Utilities Inc	Jun. 30, 2013 Northern Utilities Inc	Apr. 30, 2014 Capital Lease Obligations	Oct. 04, 2013 Revolving Credit Facility	Jun. 30, 2014 Revolving Credit Facility	Dec. 31, 2013 Revolving Credit Facility	Oct. 04, 2013 Revolving Credit Facility	Jun. 30, 2013 Revolving Credit Facility	Oct. 04, 2013 Revolving Credit Facility London Interbank Offered Rate	Jun. 30, 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	Jun. 30, 2013 8.00% Senior Secured Notes, Due Through August 1, 2017 Unitil Realty Corp	Jun. 30, 2014 8.00% Senior Secured Notes, Due Through August 1, 2017 Unitil Realty Corp Debt Outstanding, Principal Amount	Jun. 30, 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	Jun. 30, 2013 7.15% Senior Notes, Due December 15, 2018 Granite State Gas Transmission Inc	Jun. 30, 2014 7.15% Senior Notes, Due December 15, 2018 Granite State Gas Transmission Inc Debt Outstanding, Principal Amount
Line of Credit Facility [Line Items]
Revolving credit facility expiration date								Oct. 04, 2018
Line of credit facility, maximum borrowing capacity							$ 15.0		$ 120.0	$ 120.0	$ 120.0	$ 60.0
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 June 30, 2014, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
Proceeds from capital lease obligation	5.0
Natural gas storage inventory				7.8	12.5	7.2
Accounts Payable	20.5	38.1	21.7	0.2	2.7	0.1
Guarantee outstanding	28.5
Senior Secured Notes														2.1	2.3	2.6	2.1
Senior Notes																		$ 10.0	$ 10.0	$ 10.0	$ 10.0

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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).

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Carrying amount as of the balance sheet date of natural gas in storage, which is a mixture of gases (liquefied or otherwise), used for fuel and manufacturing purposes, which is ready for sale.

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Date the credit facility terminates, in CCYY-MM-DD format.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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The cash inflow from proceeds received from lessor related to a long-term capital lease obligation.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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Corporate guarantees on energy and natural gas storage management contracts entered into by the Company's distribution utilities.

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