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Regulatory Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 39.4us-gaap_RegulatoryLiabilityCurrent	$ 8.7us-gaap_RegulatoryLiabilityCurrent	$ 13.1us-gaap_RegulatoryLiabilityCurrent
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Regulatory Liabilities	17.4us-gaap_RegulatoryLiabilityCurrent / us-gaap_RegulatoryLiabilityAxis = utl_RegulatoryTrackerMechanismsMember	8.7us-gaap_RegulatoryLiabilityCurrent / us-gaap_RegulatoryLiabilityAxis = utl_RegulatoryTrackerMechanismsMember	13.1us-gaap_RegulatoryLiabilityCurrent / us-gaap_RegulatoryLiabilityAxis = utl_RegulatoryTrackerMechanismsMember
Gas Pipeline Refund
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 22.0us-gaap_RegulatoryLiabilityCurrent / us-gaap_RegulatoryLiabilityAxis = utl_GasPipelineRefundMember

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- Definition

The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities 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 405

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

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