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Components of Exchange Gas Receivable (Detail) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020 | Sep. 30, 2020
--- | --- | --- | ---
Receivables [Line Items] |  |  |
Total Exchange Gas Receivable | $ 10.5 | $ 4.9 | $ 6.5
Northern Utilities Inc |  |  |
Receivables [Line Items] |  |  |
Total Exchange Gas Receivable | 9.7 | 4.4 | 6.0
Fitchburg |  |  |
Receivables [Line Items] |  |  |
Total Exchange Gas Receivable | $ 0.8 | $ 0.5 | $ 0.5

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: |