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Summary of Significant Accounting Policies - Components of Exchange Gas Receivable (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Receivables [Line Items]
Total Exchange Gas Receivable	$ 9.3	$ 9.4	$ 14.0
Northern Utilities Inc
Receivables [Line Items]
Total Exchange Gas Receivable	8.8	8.6	13.1
Fitchburg
Receivables [Line Items]
Total Exchange Gas Receivable	$ 0.5	$ 0.8	$ 0.9

X

- Definition

Amount, after allowance, of receivables classified as other, due within one year or the operating cycle, if longer.

+ 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: