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Significant Segment Financial Data (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	$ 172.2us-gaap_RegulatedAndUnregulatedOperatingRevenue	$ 156.1us-gaap_RegulatedAndUnregulatedOperatingRevenue
Segment Profit (Loss)	13.6us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic	12.6us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Identifiable Segment Assets	1,040.8us-gaap_Assets	928.0us-gaap_Assets	1,000.2us-gaap_Assets
Capital Expenditures	13.1us-gaap_PaymentsToAcquirePropertyPlantAndEquipment	9.2us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	100.3us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember	92.6us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember
Segment Profit (Loss)	11.6us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember	11.5us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember
Identifiable Segment Assets	593.2us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember	506.1us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember	566.3us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember
Capital Expenditures	7.2us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember	3.0us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember
Electricity
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	70.3us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember	61.9us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember
Segment Profit (Loss)	1.8us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember	0.9us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember
Identifiable Segment Assets	421.3us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember	405.1us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember	414.1us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember
Capital Expenditures	4.4us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember	5.1us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)	(0.1)us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember
Identifiable Segment Assets	20.7us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember	11.0us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember	13.5us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember
Capital Expenditures	1.5us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember	1.0us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	1.6us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember	1.6us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Segment Profit (Loss)	0.3us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember	0.2us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Identifiable Segment Assets	5.6us-gaap_Assets / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember	5.8us-gaap_Assets / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember	6.3us-gaap_Assets / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Capital Expenditures		$ 0.1us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember

X

- Definition

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

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

-Publisher FASB

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-Number 210

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- Definition

Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

+ References

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+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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-URI http://asc.fasb.org/extlink&oid=6516133



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+ References

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