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Assets Measured at Fair Value on Recurring Basis for PBOP Plan (Detail) (Other Postretirement Benefit Plans, Defined Benefit, USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items] |  |
Assets measured at fair value on recurring basis | $ 12,840 us-gaap_AssetsFairValueDisclosureRecurring | $ 10,829 us-gaap_AssetsFairValueDisclosureRecurring
Fixed Income Securities |  |
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items] |  |
Assets measured at fair value on recurring basis | 5,661 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | 4,689 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Index Funds |  |
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items] |  |
Assets measured at fair value on recurring basis | 5,313 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = utl_IndexFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | 4,467 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = utl_IndexFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Equity Funds |  |
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items] |  |
Assets measured at fair value on recurring basis | 1,866 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquityFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | 1,673 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquityFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Fair Value, Inputs, Level 1 |  |
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items] |  |
Assets measured at fair value on recurring basis | 12,840 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 10,829 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Fair Value, Inputs, Level 1 | Fixed Income Securities |  |
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items] |  |
Assets measured at fair value on recurring basis | 5,661 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 4,689 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Fair Value, Inputs, Level 1 | Index Funds |  |
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items] |  |
Assets measured at fair value on recurring basis | 5,313 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = utl_IndexFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 4,467 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = utl_IndexFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Fair Value, Inputs, Level 1 | Equity Funds |  |
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items] |  |
Assets measured at fair value on recurring basis | $ 1,866 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquityFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | $ 1,673 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquityFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member

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- Definition This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a recurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References No definition available. + Details
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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |