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Significant Segment Financial Data (Detail) (USD $) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues | $ 119,800,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 76,600,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 73,300,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 156,100,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 109,800,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 72,500,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 66,400,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 118,200,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 425,800,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 366,900,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue | $ 353,100,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue
Interest Income |  |  |  |  |  |  |  |  | 1,300,000 us-gaap_InvestmentIncomeInterest | 3,200,000 us-gaap_InvestmentIncomeInterest | 4,200,000 us-gaap_InvestmentIncomeInterest
Interest Expense |  |  |  |  |  |  |  |  | 22,200,000 us-gaap_InterestExpenseDebt | 22,000,000 us-gaap_InterestExpenseDebt | 22,300,000 us-gaap_InterestExpenseDebt
Depreciation & Amortization Expense |  |  |  |  |  |  |  |  | 42,100,000 us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization | 38,500,000 us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization | 36,000,000 us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | 13,995,000 us-gaap_IncomeTaxExpenseBenefit | 12,652,000 us-gaap_IncomeTaxExpenseBenefit | 10,976,000 us-gaap_IncomeTaxExpenseBenefit
Segment Profit | 9,400,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 1,600,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 1,100,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 12,600,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 10,300,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 600,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | (100,000)us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 10,800,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 24,700,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 21,600,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 18,100,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Identifiable Segment Assets | 1,000,200,000 us-gaap_Assets |  |  |  | 920,600,000 us-gaap_Assets |  |  |  | 1,000,200,000 us-gaap_Assets | 920,600,000 us-gaap_Assets | 892,300,000 us-gaap_Assets
Capital Expenditures |  |  |  |  |  |  |  |  | 92,600,000 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment | 89,500,000 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment | 68,500,000 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
Gas Segment |  |  |  |  |  |  |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 201,400,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 170,400,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 160,600,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember
Interest Income |  |  |  |  |  |  |  |  | 300,000 us-gaap_InvestmentIncomeInterest / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 500,000 us-gaap_InvestmentIncomeInterest / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 800,000 us-gaap_InvestmentIncomeInterest / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember
Interest Expense |  |  |  |  |  |  |  |  | 11,500,000 us-gaap_InterestExpenseDebt / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 11,000,000 us-gaap_InterestExpenseDebt / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 11,100,000 us-gaap_InterestExpenseDebt / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember
Depreciation & Amortization Expense |  |  |  |  |  |  |  |  | 18,800,000 us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 17,200,000 us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 15,700,000 us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | 10,800,000 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 7,500,000 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 5,800,000 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember
Segment Profit |  |  |  |  |  |  |  |  | 15,800,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 12,500,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 8,900,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember
Identifiable Segment Assets | 566,300,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember |  |  |  | 502,300,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember |  |  |  | 566,300,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 502,300,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 471,700,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember
Capital Expenditures |  |  |  |  |  |  |  |  | 62,300,000 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 61,100,000 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember | 43,900,000 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = utl_GasSegmentMember
Electricity |  |  |  |  |  |  |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 218,700,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 190,700,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 187,000,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember
Interest Income |  |  |  |  |  |  |  |  | 600,000 us-gaap_InvestmentIncomeInterest / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 2,200,000 us-gaap_InvestmentIncomeInterest / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 2,900,000 us-gaap_InvestmentIncomeInterest / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember
Interest Expense |  |  |  |  |  |  |  |  | 9,100,000 us-gaap_InterestExpenseDebt / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 9,500,000 us-gaap_InterestExpenseDebt / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 9,500,000 us-gaap_InterestExpenseDebt / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember
Depreciation & Amortization Expense |  |  |  |  |  |  |  |  | 22,300,000 us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 20,300,000 us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 18,900,000 us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | 4,500,000 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 5,100,000 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 4,800,000 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember
Segment Profit |  |  |  |  |  |  |  |  | 6,800,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 7,600,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 7,600,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember
Identifiable Segment Assets | 414,100,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember |  |  |  | 402,800,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember |  |  |  | 414,100,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 402,800,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 409,300,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember
Capital Expenditures |  |  |  |  |  |  |  |  | 24,800,000 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 23,600,000 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember | 21,200,000 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = utl_ElectricityMember
All Other Segments |  |  |  |  |  |  |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest Income |  |  |  |  |  |  |  |  | 300,000 us-gaap_InvestmentIncomeInterest / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 400,000 us-gaap_InvestmentIncomeInterest / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 400,000 us-gaap_InvestmentIncomeInterest / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember
Interest Expense |  |  |  |  |  |  |  |  | 1,600,000 us-gaap_InterestExpenseDebt / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 1,500,000 us-gaap_InterestExpenseDebt / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 1,700,000 us-gaap_InterestExpenseDebt / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember
Depreciation & Amortization Expense |  |  |  |  |  |  |  |  | 1,000,000 us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 1,000,000 us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 1,400,000 us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | (1,900,000)us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | (700,000)us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | (500,000)us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember
Segment Profit |  |  |  |  |  |  |  |  | 1,200,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 300,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 300,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember
Identifiable Segment Assets | 13,500,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |  |  | 9,300,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |  |  | 13,500,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 9,300,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 5,600,000 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember
Capital Expenditures |  |  |  |  |  |  |  |  | 5,200,000 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 4,800,000 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 3,400,000 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember
Unregulated Operation |  |  |  |  |  |  |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 5,700,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 5,800,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 5,500,000 us-gaap_RegulatedAndUnregulatedOperatingRevenue / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Interest Income |  |  |  |  |  |  |  |  | 100,000 us-gaap_InvestmentIncomeInterest / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 100,000 us-gaap_InvestmentIncomeInterest / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 100,000 us-gaap_InvestmentIncomeInterest / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | 600,000 us-gaap_IncomeTaxExpenseBenefit / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 800,000 us-gaap_IncomeTaxExpenseBenefit / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 900,000 us-gaap_IncomeTaxExpenseBenefit / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Segment Profit |  |  |  |  |  |  |  |  | 900,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 1,200,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 1,300,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Identifiable Segment Assets | 6,300,000 us-gaap_Assets / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember |  |  |  | 6,200,000 us-gaap_Assets / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember |  |  |  | 6,300,000 us-gaap_Assets / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 6,200,000 us-gaap_Assets / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 5,700,000 us-gaap_Assets / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Capital Expenditures |  |  |  |  |  |  |  |  | $ 300,000 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember |  |

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