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Regulatory Assets Liabilities and Reclassification from Regulatory Assets Liabilities into Purchased Gas (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012
Natural Gas Future Contract
Regulatory Assets [Line Items]
Loss (Gain) recognized in Regulatory Assets	$ 0.1		$ (0.7)	$ 0.7		$ 0.7
Gas Purchase Costs
Regulatory Assets [Line Items]
Loss Reclassified into unaudited Consolidated Statement of Earnings	$ 0.2	[1]		$ 1.1	[1]	$ 2.2	[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

Amount of realized and unrealized loss of derivative instruments not designated or qualifying 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

+ Details

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X

- Details

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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_
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