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Retirement Benefit Plans - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]        
Change in Discount Rate 0.25%utl_ChangeInDiscountRateAssumption      
Increase or decrease of Net Periodic Benefit Cost (NPBC) due to change in the discount rate $ 341,000us-gaap_DefinedBenefitPlanEffectOfPlanAmendmentOnNetPeriodicBenefitCost      
Pension expense 4,900,000us-gaap_PensionExpense 6,600,000us-gaap_PensionExpense 6,300,000us-gaap_PensionExpense  
Regulatory Assets 145,400,000us-gaap_RegulatoryAssets 143,700,000us-gaap_RegulatoryAssets    
Defined Benefit Plan, Expected Long-term Return on Assets 8.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets      
Defined Benefit Plan, Expected Long-term Rate-of-Return on Assets Assumption The desired investment objective is a long-term rate of return on assets that is approximately 5 - 6% greater than the assumed rate of inflation as measured by the Consumer Price Index.      
Defined Benefit Obligations        
Defined Benefit Plan Disclosure [Line Items]        
Regulatory Assets 65,100,000us-gaap_RegulatoryAssets
/ utl_DefinedBenefitPensionPlanAxis
= utl_DefinedBenefitObligationsMember		 42,600,000us-gaap_RegulatoryAssets
/ utl_DefinedBenefitPensionPlanAxis
= utl_DefinedBenefitObligationsMember		    
Fair Value Of Plan Assets        
Defined Benefit Plan Disclosure [Line Items]        
Pension expense 4,300,000us-gaap_PensionExpense
/ utl_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis
= utl_FairValueOfPlanAssetsMember		 6,600,000us-gaap_PensionExpense
/ utl_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis
= utl_FairValueOfPlanAssetsMember		 6,700,000us-gaap_PensionExpense
/ utl_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis
= utl_FairValueOfPlanAssetsMember		  
Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Accumulated Benefit Obligation 121,800,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 96,900,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		    
Company's contributions 4,191,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 3,700,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 9,387,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Other Postretirement Benefit Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Company's contributions 3,650,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		 3,280,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		 2,190,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember		  
Supplemental Employee Retirement Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Accumulated Benefit Obligation 6,300,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember		 5,100,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember		    
Company's contributions 53,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember		 53,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember		 53,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember		  
Benefit Obligation        
Defined Benefit Plan Disclosure [Line Items]        
Increase or decrease of Net Periodic Benefit Cost (NPBC) due to change in the discount rate 28,400,000us-gaap_DefinedBenefitPlanEffectOfPlanAmendmentOnNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= utl_BenefitObligationMember		 17,000,000us-gaap_DefinedBenefitPlanEffectOfPlanAmendmentOnNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= utl_BenefitObligationMember		    
Assumed Discount Rate 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= utl_BenefitObligationMember		 4.80%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= utl_BenefitObligationMember		 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= utl_BenefitObligationMember		  
Four Zero One K Plan        
Defined Benefit Plan Disclosure [Line Items]        
Company's contributions 1,877,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= utl_FourZeroOneKPlanMember		 1,678,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= utl_FourZeroOneKPlanMember		 1,387,000us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= utl_FourZeroOneKPlanMember		  
Scenario Forecast | Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Amortizations related to Actuarial Loss and Prior Service Cost 4,900,000us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		      
Scenario Forecast | Pension Plans | Equity Funds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       48.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquityFundsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
Scenario Forecast | Pension Plans | Fixed Income Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       37.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
Scenario Forecast | Pension Plans | Real Estate Funds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       10.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_RealEstateFundsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
Scenario Forecast | Pension Plans | Equity And Debt Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       5.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= utl_EquityAndDebtSecuritiesMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		 [1]
Scenario Forecast | Other Postretirement Benefit Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Amortizations related to Actuarial Loss and Prior Service Cost 2,800,000us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		      
Scenario Forecast | Other Postretirement Benefit Plans, Defined Benefit | Equity Funds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       55.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquityFundsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
Scenario Forecast | Other Postretirement Benefit Plans, Defined Benefit | Fixed Income Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Target Plan Asset Allocations       45.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeSecuritiesMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
Scenario Forecast | Supplemental Employee Retirement Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Amortizations related to Actuarial Loss and Prior Service Cost $ 100,000us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		      
Adjustment | Benefit Obligation        
Defined Benefit Plan Disclosure [Line Items]        
Assumed Discount Rate 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= utl_BenefitObligationMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioAdjustmentMember		 4.80%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= utl_BenefitObligationMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioAdjustmentMember		    
[1] Represents investments in an asset allocation fund. This fund invests in both equity and debt securities.