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Retirement Benefit Obligations - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to benefit plan	$ 3,700,000
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Contributions to benefit plan	2,500,000
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Contributions to benefit plan	40,000
Expected additional contribution to SERP plan	$ 13,000

X

- Definition

The increase in the fair value of plan assets from contributions made by the employer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)(3)

-URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Defined Benefit Plan, Estimated Additional Employer Contributions in Remainder Of Fiscal Year

+ References

No definition available.

+ Details

Name:	utl_DefinedBenefitPlanEstimatedAdditionalEmployerContributionsInRemainderOfFiscalYear
Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant