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Exchange Gas Receivable (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Receivables [Line Items]
Total Exchange Gas Receivable	$ 4.0us-gaap_OtherReceivablesNetCurrent	$ 15.0us-gaap_OtherReceivablesNetCurrent	$ 1.3us-gaap_OtherReceivablesNetCurrent
Northern Utilities Inc
Receivables [Line Items]
Total Exchange Gas Receivable	3.6us-gaap_OtherReceivablesNetCurrent / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember	14.2us-gaap_OtherReceivablesNetCurrent / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember	1.0us-gaap_OtherReceivablesNetCurrent / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Fitchburg Gas and Electric Light Company
Receivables [Line Items]
Total Exchange Gas Receivable	$ 0.4us-gaap_OtherReceivablesNetCurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember	$ 0.8us-gaap_OtherReceivablesNetCurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember	$ 0.3us-gaap_OtherReceivablesNetCurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember

X

- Definition

The carrying amount of other receivables, net, due within one year of the balance sheet date (or one operating cycle, if longer) from third parties or arising from transactions not separately disclosed.

+ References

No definition available.

+ Details

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X

- Details

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Namespace Prefix:
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X

- Details

Name:	dei_LegalEntityAxis=utl_NorthernUtilitiesIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: