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Restricted Stock Units Issued (Detail) (Restricted Stock, USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Restricted Stock
Restricted Stock Units
Beginning Restricted Stock Units	14,903utl_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	3,883utl_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Restricted Stock Units Granted	9,078us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	10,710us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Dividend Equivalents Earned	701utl_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsDividendEquivalentRightsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	310utl_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsDividendEquivalentRightsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Restricted Stock Units Settled	(1,106)utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedSettled / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Ending Restricted Stock Units	23,576utl_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	14,903utl_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Weighted-Average Stock Price
Beginning Restricted Stock Units	$ 28.90utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 27.39utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Restricted Stock Units Granted	$ 31.23utl_ShareBasedCompensationArrangementByShareBbasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 29.43utl_ShareBasedCompensationArrangementByShareBbasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Dividend Equivalents Earned	$ 33.18utl_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedDividendEquivalentRightsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 29.47utl_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedDividendEquivalentRightsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Restricted Stock Units Settled	$ 29.49utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedSettledInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Ending Restricted Stock Units	$ 29.90utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 28.90utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember

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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(iii)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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