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Exchange Gas Receivable (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Receivables [Line Items]    
Total Exchange Gas Receivable $ 15.0us-gaap_OtherReceivablesNetCurrent $ 10.8us-gaap_OtherReceivablesNetCurrent
Northern Utilities Inc    
Receivables [Line Items]    
Total Exchange Gas Receivable 14.2us-gaap_OtherReceivablesNetCurrent
/ dei_LegalEntityAxis
= utl_NorthernUtilitiesIncMember		 9.8us-gaap_OtherReceivablesNetCurrent
/ dei_LegalEntityAxis
= utl_NorthernUtilitiesIncMember
Fitchburg Gas and Electric Light Company    
Receivables [Line Items]    
Total Exchange Gas Receivable $ 0.8us-gaap_OtherReceivablesNetCurrent
/ dei_LegalEntityAxis
= utl_FitchburgGasAndElectricLightCompanyMember		 $ 1.0us-gaap_OtherReceivablesNetCurrent
/ dei_LegalEntityAxis
= utl_FitchburgGasAndElectricLightCompanyMember