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Exchange Gas Receivable (Detail) - USD ($) $ in Millions | Mar. 31, 2016 | Dec. 31, 2015 | Mar. 31, 2015
--- | --- | --- | ---
Receivables [Line Items] |  |  |
Total Exchange Gas Receivable | $ 6.8 | $ 11.1 | $ 4.0
Northern Utilities Inc |  |  |
Receivables [Line Items] |  |  |
Total Exchange Gas Receivable | 6.3 | 10.3 | 3.6
Fitchburg Gas and Electric Light Company |  |  |
Receivables [Line Items] |  |  |
Total Exchange Gas Receivable | $ 0.5 | $ 0.8 | $ 0.4

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: |