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Environmental Matters - Additional Information (Detail) - Environmental Restoration Costs | 9 Months Ended
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Sep. 30, 2018
Maine |
Site Contingency [Line Items] |
Amortization period for environmental costs | 5 years
New Hampshire |
Site Contingency [Line Items] |
Amortization period for environmental costs | 7 years

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Remaining Amortization Period. + References No definition available. + Details
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Namespace Prefix: | utl_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RegulatoryLiabilityAxis=us-gaap_EnvironmentalRestorationCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_StatementGeographicalAxis=stpr_NH
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |