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Long Term Debt, Credit Arrangements and Guarantees - Additional Information (Detail) (USD $) In Millions, unless otherwise specified							6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Jun. 30, 2012 Northern Utilities Inc	Dec. 31, 2011 Northern Utilities Inc	Jun. 30, 2011 Northern Utilities Inc	Jun. 30, 2012 Revolving Facility	Dec. 31, 2011 Revolving Facility	Dec. 30, 2011 Revolving Facility	Jun. 30, 2011 Revolving Facility	Jul. 24, 2012 Revolving Facility Scenario, Forecast	Jun. 30, 2012 Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen Unitil Realty Corp	Jun. 30, 2012 Senior Notes Seven Point One Five Percent Due December Fifteen Twenty Eighteen Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Revolving credit facility expiration date							Oct. 08, 2013
Revolving credit facility							$ 115.0		$ 115.0	$ 80.0	$ 115.0
Reduced Borrowing Limit											60
Percentage of capitalization							65.00%	65.00%		65.00%
Outstanding Inventory				8.0	14.9	6.6
Accounts Payable	15.9	26.4	15.8	0	2.5	0
Guarantee outstanding	20.6
Guarantee end date	Feb. 28, 2014
Principal amount of notes outstanding												$ 3.1	$ 10.0

X

- Definition

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).

+ References

Reference 1: 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



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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Subparagraph a

-Article 5

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- Definition

Certain changes made in the current period to the value and amount of inventory reported.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 330

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6386783&loc=d3e4492-108314



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

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 43

-Section A

-Paragraph 9

-Chapter 3

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.6(b))

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



Reference 4: http://www.xbrl.org/2003/role/presentationRef

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-Name Regulation S-X (SX)

-Number 210

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X

- Definition

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

+ References

Reference 1: 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(b),22(b))

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



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

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-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

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- Definition

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.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



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(b),22(b))

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



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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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- Definition

Capitalization Rate

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- Definition

Debt Outstanding, Principal Amount

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- Definition

Letter Of Credit Facility And Bank Guarantees Outstanding

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- Definition

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