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Provisions for Federal and State Income Taxes (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred Federal Tax Provision (Benefit)
Deferred Federal Tax Provision (Benefit)	$ 10,809us-gaap_DeferredFederalIncomeTaxExpenseBenefit	$ 10,052us-gaap_DeferredFederalIncomeTaxExpenseBenefit	$ 9,085us-gaap_DeferredFederalIncomeTaxExpenseBenefit
Total Federal Tax Provision	10,809us-gaap_FederalIncomeTaxExpenseBenefitContinuingOperations	10,052us-gaap_FederalIncomeTaxExpenseBenefitContinuingOperations	9,085us-gaap_FederalIncomeTaxExpenseBenefitContinuingOperations
State
Current	(387)us-gaap_CurrentStateAndLocalTaxExpenseBenefit	386us-gaap_CurrentStateAndLocalTaxExpenseBenefit	132us-gaap_CurrentStateAndLocalTaxExpenseBenefit
Deferred	3,573us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit	2,214us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit	1,759us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit
Total State Tax Provision	3,186us-gaap_StateAndLocalIncomeTaxExpenseBenefitContinuingOperations	2,600us-gaap_StateAndLocalIncomeTaxExpenseBenefitContinuingOperations	1,891us-gaap_StateAndLocalIncomeTaxExpenseBenefitContinuingOperations
Total Provision for Federal and State Income Taxes	13,995us-gaap_IncomeTaxExpenseBenefit	12,652us-gaap_IncomeTaxExpenseBenefit	10,976us-gaap_IncomeTaxExpenseBenefit
Operating Income
Current Federal Tax Provision (Benefit)
Current Benefit of Operating Loss Carrybacks	3,179us-gaap_CurrentFederalTaxExpenseBenefit / utl_IncomeTaxesAxis = us-gaap_OperatingIncomeLossMember
Current Benefit of Operating Loss Carrybacks
Current Federal Tax Provision (Benefit)
Current Benefit of Operating Loss Carrybacks	(3,179)us-gaap_CurrentFederalTaxExpenseBenefit / utl_IncomeTaxesAxis = utl_OperatingLossCarrybacksMember
Utility Plant Differences
Deferred Federal Tax Provision (Benefit)
Deferred Federal Tax Provision (Benefit)	10,649us-gaap_DeferredFederalIncomeTaxExpenseBenefit / utl_IncomeTaxesAxis = utl_UtilityPlantDifferencesMember	28,907us-gaap_DeferredFederalIncomeTaxExpenseBenefit / utl_IncomeTaxesAxis = utl_UtilityPlantDifferencesMember	6,019us-gaap_DeferredFederalIncomeTaxExpenseBenefit / utl_IncomeTaxesAxis = utl_UtilityPlantDifferencesMember
Net Operating Loss Carryforwards
Deferred Federal Tax Provision (Benefit)
Deferred Federal Tax Provision (Benefit)	2,589us-gaap_DeferredFederalIncomeTaxExpenseBenefit / utl_IncomeTaxesAxis = utl_NetOperatingLossCarryforwardsMember	(8,053)us-gaap_DeferredFederalIncomeTaxExpenseBenefit / utl_IncomeTaxesAxis = utl_NetOperatingLossCarryforwardsMember	2,835us-gaap_DeferredFederalIncomeTaxExpenseBenefit / utl_IncomeTaxesAxis = utl_NetOperatingLossCarryforwardsMember
Regulatory Assets and Liabilities
Deferred Federal Tax Provision (Benefit)
Deferred Federal Tax Provision (Benefit)	(5,946)us-gaap_DeferredFederalIncomeTaxExpenseBenefit / utl_IncomeTaxesAxis = utl_RegulatoryAssetsAndLiabilitiesMember	(11,483)us-gaap_DeferredFederalIncomeTaxExpenseBenefit / utl_IncomeTaxesAxis = utl_RegulatoryAssetsAndLiabilitiesMember	472us-gaap_DeferredFederalIncomeTaxExpenseBenefit / utl_IncomeTaxesAxis = utl_RegulatoryAssetsAndLiabilitiesMember
Other, net
Deferred Federal Tax Provision (Benefit)
Deferred Federal Tax Provision (Benefit)	$ 3,517us-gaap_DeferredFederalIncomeTaxExpenseBenefit / utl_IncomeTaxesAxis = utl_OtherNetMember	$ 681us-gaap_DeferredFederalIncomeTaxExpenseBenefit / utl_IncomeTaxesAxis = utl_OtherNetMember	$ (241)us-gaap_DeferredFederalIncomeTaxExpenseBenefit / utl_IncomeTaxesAxis = utl_OtherNetMember

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