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Retirement Benefit Plans - Additional Information (Detail) (USD $) | 12 Months Ended
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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 | $ 341,000 us-gaap_DefinedBenefitPlanEffectOfPlanAmendmentOnNetPeriodicBenefitCost |  |  |  |
Pension expense | 4,900,000 us-gaap_PensionExpense | 6,600,000 us-gaap_PensionExpense | 6,300,000 us-gaap_PensionExpense |  |
Regulatory Assets | 145,400,000 us-gaap_RegulatoryAssets | 143,700,000 us-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 | 65,100,000 us-gaap_RegulatoryAssets / utl_DefinedBenefitPensionPlanAxis = utl_DefinedBenefitObligationsMember | 42,600,000 us-gaap_RegulatoryAssets / utl_DefinedBenefitPensionPlanAxis = utl_DefinedBenefitObligationsMember |  |  |
Fair Value Of Plan Assets |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Pension expense | 4,300,000 us-gaap_PensionExpense / utl_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = utl_FairValueOfPlanAssetsMember | 6,600,000 us-gaap_PensionExpense / utl_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = utl_FairValueOfPlanAssetsMember | 6,700,000 us-gaap_PensionExpense / utl_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = utl_FairValueOfPlanAssetsMember |  |
Pension Plans |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Accumulated Benefit Obligation | 121,800,000 us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 96,900,000 us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  |  |
Company's contributions | 4,191,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 3,700,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 9,387,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  |
Other Postretirement Benefit Plans, Defined Benefit |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Company's contributions | 3,650,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | 3,280,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | 2,190,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  |
Supplemental Employee Retirement Plans, Defined Benefit |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Accumulated Benefit Obligation | 6,300,000 us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember | 5,100,000 us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember |  |  |
Company's contributions | 53,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember | 53,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember | 53,000 us-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 | 28,400,000 us-gaap_DefinedBenefitPlanEffectOfPlanAmendmentOnNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = utl_BenefitObligationMember | 17,000,000 us-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 | 1,877,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = utl_FourZeroOneKPlanMember | 1,678,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = utl_FourZeroOneKPlanMember | 1,387,000 us-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 | 4,900,000 us-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 | 2,800,000 us-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 | $ 100,000 us-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.

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- Definition For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503844 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The interest rate used to adjust for the time value of money. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Expected Long-Term Rate of Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512013 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The increase in the fair value of plan assets from contributions made by the employer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The amount of increase or decrease in net periodic benefit cost related to the effects of a change in the substantive terms of an existing defined benefit plan or the initiation of a new plan. A plan amendment may increase or decrease benefits, including those attributed to years of service already rendered. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Glossary Plan Amendment -URI http://asc.fasb.org/extlink&oid=6520743 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Plan Amendment -URI http://asc.fasb.org/extlink&oid=6520782 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (r) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition A narrative description of the basis used to determine the overall expected long-term rate-of-return on assets assumption, such as the general approach used, the extent to which the overall rate-of-return on assets assumption was based on historical returns, the extent to which adjustments were made to those historical returns to reflect expectations of future returns, and how those adjustments were determined. The description would generally consider the major categories of assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition Amount of employer's best estimate of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition Target allocation of investments of this type to total plan assets presented on a weighted average basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920 + Details
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- Definition The amount of pension benefit costs recognized during the period for (1) defined benefit plans and (2) defined contribution plans. For defined benefit plans, pension expense includes the following components: service cost, interest cost, expected return on plan assets, gain (loss) on plan assets, prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. For defined contribution plans, the pension expense generally equals the firm's contribution to employees' accounts (if the firm contributes) during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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