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Regulatory Assets (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Regulatory Assets [Line Items]    
Regulatory Assets $ 145.4us-gaap_RegulatoryAssets $ 143.7us-gaap_RegulatoryAssets
Less: Current Portion of Regulatory Assets 37.8us-gaap_RegulatoryAssetsCurrent [1] 43.6us-gaap_RegulatoryAssetsCurrent [1]
Regulatory Assets-noncurrent 107.6us-gaap_RegulatoryAssetsNoncurrent 100.1us-gaap_RegulatoryAssetsNoncurrent
Environmental Matters    
Regulatory Assets [Line Items]    
Regulatory Assets 11.0us-gaap_RegulatoryAssets
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= utl_EnvironmentalMattersMember
16.1us-gaap_RegulatoryAssets
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= utl_EnvironmentalMattersMember
Retirement Benefits    
Regulatory Assets [Line Items]    
Regulatory Assets 65.1us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= utl_PostretirementBenefitMember
42.6us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= utl_PostretirementBenefitMember
Deferred Storm Charges    
Regulatory Assets [Line Items]    
Regulatory Assets 21.2us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= utl_ElectricUtilitiesMember
25.6us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= utl_ElectricUtilitiesMember
Income Taxes    
Regulatory Assets [Line Items]    
Regulatory Assets 9.7us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= utl_IncomeTaxesMember
11.9us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= utl_IncomeTaxesMember
Other Assets    
Regulatory Assets [Line Items]    
Regulatory Assets 5.8us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_OtherAssetsMember
5.7us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_OtherAssetsMember
Energy Supply & Other Regulatory Tracker Mechanisms    
Regulatory Assets [Line Items]    
Regulatory Assets 31.0us-gaap_RegulatoryAssets
/ us-gaap_RestructuringCostAndReserveAxis
= utl_ContractualObligationsMember
32.5us-gaap_RegulatoryAssets
/ us-gaap_RestructuringCostAndReserveAxis
= utl_ContractualObligationsMember
Deferred Restructuring Costs    
Regulatory Assets [Line Items]    
Regulatory Assets $ 1.6us-gaap_RegulatoryAssets
/ us-gaap_RestructuringCostAndReserveAxis
= utl_RestructuringCostsMember
$ 9.3us-gaap_RegulatoryAssets
/ us-gaap_RestructuringCostAndReserveAxis
= utl_RestructuringCostsMember
[1] Reflects amounts included in Accrued Revenue on the Company's Consolidated Balance Sheets and in the Accrued Revenue table shown above