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Components of Accrued Revenue (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Deferred Revenue Arrangement [Line Items]
Accrued Revenue	$ 64.2us-gaap_UnbilledContractsReceivable	$ 48.5us-gaap_UnbilledContractsReceivable	$ 49.6us-gaap_UnbilledContractsReceivable
Gas Pipeline Refund
Deferred Revenue Arrangement [Line Items]
Accrued Revenue	22.0us-gaap_UnbilledContractsReceivable / us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis = utl_GasPipelineRefundMember
Unbilled Revenues
Deferred Revenue Arrangement [Line Items]
Accrued Revenue	8.5us-gaap_UnbilledContractsReceivable / us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis = us-gaap_UnbilledRevenuesMember	10.7us-gaap_UnbilledContractsReceivable / us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis = us-gaap_UnbilledRevenuesMember	9.9us-gaap_UnbilledContractsReceivable / us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis = us-gaap_UnbilledRevenuesMember
Regulatory Asset
Deferred Revenue Arrangement [Line Items]
Accrued Revenue	$ 33.7us-gaap_UnbilledContractsReceivable / us-gaap_BalanceSheetLocationAxis = utl_RegulatoryAssetMember	$ 37.8us-gaap_UnbilledContractsReceivable / us-gaap_BalanceSheetLocationAxis = utl_RegulatoryAssetMember	$ 39.7us-gaap_UnbilledContractsReceivable / us-gaap_BalanceSheetLocationAxis = utl_RegulatoryAssetMember

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- Definition

Unbilled amounts due for services rendered or to be rendered, actions taken or to be taken, or a promise to refrain from taking certain actions in accordance with the terms of a legally binding agreement between the entity and, at a minimum, one other party. An example would be amounts associated with contracts or programs where the recognized revenue for performance thereunder exceeds the amounts billed under the terms thereof as of the date of the balance sheet.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.3(c)(3))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 3

-Subparagraph c(3)

-Article 5



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 912

-SubTopic 310

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6471842&loc=d3e55302-109406

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