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Regulatory Assets (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
Regulatory Assets [Line Items]
Regulatory Assets	$ 139.9us-gaap_RegulatoryAssets		$ 145.4us-gaap_RegulatoryAssets		$ 136.2us-gaap_RegulatoryAssets
Less: Current Portion of Regulatory Assets	33.7us-gaap_RegulatoryAssetsCurrent	[1]	37.8us-gaap_RegulatoryAssetsCurrent	[1]	39.7us-gaap_RegulatoryAssetsCurrent	[1]
Regulatory Assets - noncurrent	106.2us-gaap_RegulatoryAssetsNoncurrent		107.6us-gaap_RegulatoryAssetsNoncurrent		96.5us-gaap_RegulatoryAssetsNoncurrent
Environmental Matters
Regulatory Assets [Line Items]
Regulatory Assets	11.0us-gaap_RegulatoryAssets / us-gaap_LossContingenciesByNatureOfContingencyAxis = utl_EnvironmentalMattersMember		11.0us-gaap_RegulatoryAssets / us-gaap_LossContingenciesByNatureOfContingencyAxis = utl_EnvironmentalMattersMember		16.0us-gaap_RegulatoryAssets / us-gaap_LossContingenciesByNatureOfContingencyAxis = utl_EnvironmentalMattersMember
Retirement Benefits
Regulatory Assets [Line Items]
Regulatory Assets	65.0us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = utl_PostretirementBenefitMember		65.1us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = utl_PostretirementBenefitMember		42.3us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = utl_PostretirementBenefitMember
Deferred Storm Charges
Regulatory Assets [Line Items]
Regulatory Assets	20.1us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = utl_ElectricUtilitiesMember		21.2us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = utl_ElectricUtilitiesMember		24.2us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = utl_ElectricUtilitiesMember
Income Taxes
Regulatory Assets [Line Items]
Regulatory Assets	9.5us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = utl_IncomeTaxesMember		9.7us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = utl_IncomeTaxesMember		10.7us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = utl_IncomeTaxesMember
Other Assets
Regulatory Assets [Line Items]
Regulatory Assets	5.8us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_OtherAssetsMember		5.8us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_OtherAssetsMember		5.8us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_OtherAssetsMember
Energy Supply & Other Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Regulatory Assets	28.5us-gaap_RegulatoryAssets / us-gaap_RestructuringCostAndReserveAxis = utl_ContractualObligationsMember		31.0us-gaap_RegulatoryAssets / us-gaap_RestructuringCostAndReserveAxis = utl_ContractualObligationsMember		30.3us-gaap_RegulatoryAssets / us-gaap_RestructuringCostAndReserveAxis = utl_ContractualObligationsMember
Deferred Restructuring Costs
Regulatory Assets [Line Items]
Regulatory Assets			$ 1.6us-gaap_RegulatoryAssets / us-gaap_RestructuringCostAndReserveAxis = utl_RestructuringCostsMember		$ 6.9us-gaap_RegulatoryAssets / us-gaap_RestructuringCostAndReserveAxis = utl_RestructuringCostsMember
[1] Reflects amounts included in Accrued Revenue, discussed above, on the Company's Consolidated Balance Sheets.

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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

+ References

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-Section 50

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-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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