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Components of Exchange Gas Receivable (Detail) - USD ($) $ in Millions | Mar. 31, 2023 | Dec. 31, 2022 | Mar. 31, 2022
--- | --- | --- | ---
Receivables [Line Items] |  |  |
Total Exchange Gas Receivable | $ 4.7 | $ 18.0 | $ 0.7
Northern Utilities Inc |  |  |
Receivables [Line Items] |  |  |
Total Exchange Gas Receivable | 4.0 | 16.3 | 0.5
Fitchburg |  |  |
Receivables [Line Items] |  |  |
Total Exchange Gas Receivable | $ 0.7 | $ 1.7 | $ 0.2

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: |