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Deferred Tax Assets and Liabilities Current, Non Current (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Deferred Tax Assets and Liabilities [Line Items]
Utility Plant Differences	$ 120,534us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment	$ 102,479us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment
Retirement Benefit Obligations	(44,829)utl_DeferredTaxAssetsPensionAndPostretirementBenefits	(28,287)utl_DeferredTaxAssetsPensionAndPostretirementBenefits
Net Operating Loss Carryforwards	(13,122)us-gaap_DeferredTaxAssetsOperatingLossCarryforwards	(17,403)us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
Regulatory Assets & Liabilities	12,740us-gaap_DeferredTaxAssetsNet	17,174us-gaap_DeferredTaxAssetsNet
AMT Tax Credit Carryforwards	(2,139)us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax	(1,538)us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax
Other, net	(314)utl_DeferredTaxAssetsLiabilitiesOther	754utl_DeferredTaxAssetsLiabilitiesOther
Total Noncurrent Deferred Income Tax Liabilities	$ 72,870us-gaap_DeferredTaxLiabilitiesNoncurrent	$ 73,179us-gaap_DeferredTaxLiabilitiesNoncurrent

X

- Definition

Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b),(c)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319

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X

- Definition

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319



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-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

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-Section 25

-Paragraph 20

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- Definition

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible alternative minimum tax credit carryforwards.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319



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-Topic 740

-SubTopic 10

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Alternative Minimum Tax

-URI http://asc.fasb.org/extlink&oid=6504908

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- Definition

Amount of deferred tax liability attributable to taxable temporary differences, net of deferred tax asset attributable to deductible temporary differences and carryforwards net of valuation allowances expected to be realized or consumed after one year (or the normal operating cycle, if longer).

+ References

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- Definition

Amount of deferred tax liability attributable to taxable temporary differences from property, plant, and equipment.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

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