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Environmental Matters - Additional Information (Detail) - Environmental Restoration Costs	3 Months Ended
	Mar. 31, 2019
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

X

- 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_
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Balance Type:	na
Period Type:	duration

X

- Definition

Remaining Amortization Period.

+ References

No definition available.

+ Details

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Namespace Prefix:	utl_
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Balance Type:	na
Period Type:	duration

X

- Details

Name:	srt_StatementGeographicalAxis=stpr_ME
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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

X

- Details

Name:	srt_StatementGeographicalAxis=stpr_NH
Namespace Prefix:
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Period Type: