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Exchange Gas Receivable (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Receivables [Line Items]
Total Exchange Gas Receivable	$ 10.6	$ 15.0	$ 14.5
Northern Utilities Inc
Receivables [Line Items]
Total Exchange Gas Receivable	9.9	14.2	13.4
Fitchburg Gas and Electric Light Company
Receivables [Line Items]
Total Exchange Gas Receivable	$ 0.7	$ 0.8	$ 1.1

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

Name:	us-gaap_OtherReceivablesNetCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Line items represent receivables.

+ References

No definition available.

+ Details

Name:	utl_ReceivablesLineItems
Namespace Prefix:	utl_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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

X

- Details

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