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Fair Value of Pension Plan Investments Estimated using Net Asset Value (Detail) (SEI Core Property Collective Investment Trust Fund, USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
SEI Core Property Collective Investment Trust Fund |  |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |
Fair Value | $ 8,172,378 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = utl_SeiCorePropertyCollectiveInvestmentTrustFundMember | [1] | $ 1,125,000 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = utl_SeiCorePropertyCollectiveInvestmentTrustFundMember | [1]
Unfunded Commitment | $ 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = utl_SeiCorePropertyCollectiveInvestmentTrustFundMember | [1] | $ 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = utl_SeiCorePropertyCollectiveInvestmentTrustFundMember | [1]
Redemption on Frequency | Quarterly | [1] | Quarterly | [1]
Redemption on Notice Period | 65 days | [1] | 65 days | [1]
[1] | The SEI Core Property Collective Investment Trust Fund, through the SEI Core Property Fund, seeks both current income and long-term capital appreciation through investing in underlying funds that acquire, manage, and dispose of commercial real estate properties.

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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + 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) -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 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=29635418&loc=d3e12355-114930 + Details
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- Definition States the frequency (for example, annually, semi-annually, quarterly) with which the entity can redeem its investment, or portion thereof, for each of those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments), by major category. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=36462937&loc=SL6742756-110258 + Details
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- Definition Notice period the entity is required to deliver before it can redeem an investment calculated using net asset value per share, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, investments calculated by per unit, per membership interest, other equity or ownership interest and alternative investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=36462937&loc=SL6742756-110258 + Details
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- Definition The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=36462937&loc=SL6742756-110258 + Details
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