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Income Taxes - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended 3 Months Ended
Dec. 31, 2014
Mar. 31, 2015
Operating Loss Carryforwards [Line Items]    
Alternative Minimum tax credit carryforwards 2.1us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax  
Internal Revenue Service (IRS)    
Operating Loss Carryforwards [Line Items]    
NOL carryforward assets expiration date 2029  
Federal    
Operating Loss Carryforwards [Line Items]    
Tax examination description   The Company remains subject to examination by Federal, Maine, Massachusetts, and New Hampshire tax authorities for the tax periods ended December 31, 2011; December 31, 2012; and December 31, 2013.
Federal and State    
Operating Loss Carryforwards [Line Items]    
Net Operating Loss carryforward assets 13.1utl_NetOperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= utl_FederalAndStateMember		  
State and Local Jurisdiction    
Operating Loss Carryforwards [Line Items]    
NOL carryforward assets expiration date 2019  
Federal Tax | Internal Revenue Service (IRS)    
Operating Loss Carryforwards [Line Items]    
Net Operating Loss carryforward assets 2.1utl_NetOperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= utl_FederalTaxMember
/ us-gaap_IncomeTaxAuthorityNameAxis
= us-gaap_InternalRevenueServiceIRSMember