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).

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Subparagraph a

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(a))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Debt and Financing Arrangements - Additional Information (Detail) (USD $)	12 Months Ended								1 Months Ended	12 Months Ended		1 Months Ended							12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Unitil Corporation Maximum	Dec. 31, 2013 Fitchburg Gas and Electric Light Company	Dec. 31, 2013 Northern Utilities Inc	Dec. 31, 2012 Northern Utilities Inc	Dec. 31, 2013 Granite State Gas Transmission Inc	Oct. 04, 2013 Revolving Credit Facility	Dec. 31, 2013 Revolving Credit Facility	Dec. 31, 2012 Revolving Credit Facility	Oct. 04, 2013 Revolving Credit Facility London Interbank Offered Rate	Dec. 31, 2013 8.00% Senior Secured Notes, Due August 1, 2017 Unitil Realty Corp	Dec. 31, 2012 8.00% Senior Secured Notes, Due August 1, 2017 Unitil Realty Corp	Dec. 31, 2013 8.00% Senior Secured Notes, Due August 1, 2017 Unitil Realty Corp Debt Outstanding, Principal Amount	Dec. 31, 2013 7.15% Senior Notes, Due December 15, 2018 Granite State Gas Transmission Inc	Dec. 31, 2012 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 Debt Outstanding, Principal Amount	Dec. 31, 2013 Bonds	Dec. 31, 2012 Bonds	Dec. 31, 2011 Bonds
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization				70.00%	65.00%	65.00%		65.00%
Long term debt repayments	$ 500,000	$ 500,000	$ 500,000																$ 541,938	$ 500,405	$ 462,055
Debt repayment, 2014																			2,486,919
Debt repayment, 2015																			4,035,633
Debt repayment, 2016																			17,421,724
Debt repayment, 2017																			17,160,985
Debt repayment, 2018																			30,133,332
Revolving credit facility expiration date									Oct. 04, 2018
Revolving credit facility									120,000,000	120,000,000	60,000,000
Sublimit for the issuance of standby letters of credit									25,000,000
Credit facility, daily fluctuating rate of interest												1.375%
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 December 31, 2013, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
Weighted average interest rate on short term borrowings	1.80%	2.00%	2.20%
Outstanding inventory						12,500,000	10,700,000
Accounts Payable	38,100,000	32,700,000				2,700,000	2,100,000
Total rental expense under operating leases	1,200,000	1,300,000	1,400,000
Guarantee outstanding	19,100,000
Guarantee end date	Oct. 31, 2014
Senior Secured Notes													2,300,000	2,800,000	2,300,000
Senior Notes																$ 10,000,000	$ 10,000,000	$ 10,000,000