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Regulatory Assets (Liabilities) and Reclassification from Regulatory Asset (Liabilities) into Purchased Gas (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Natural Gas Future Contract
Regulatory Assets [Line Items]
Loss (Gain) recognized in Regulatory Assets (Liabilities)	$ (0.3)		$ 1.3
Gas Purchase Costs
Regulatory Assets [Line Items]
Loss Reclassified into unaudited Consolidated Statement of Earnings	$ 0.9	[1]	$ 1.6	[1]
[1] These amounts are offset in the unaudited Consolidated Statements of Earnings with Accrued Revenue and therefore there is no effect on earnings.

X

- Definition

Loss included in earnings for the period from the change in fair value of derivative instruments not designated as hedging instruments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959



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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 45

-Subparagraph b(1)(b)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Derivative Instruments Gain Loss Recognized In Regulatory Assets

+ References

No definition available.

+ Details

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Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration