XML 60 R46.htm IDEA: XBRL DOCUMENT
v3.8.0.1

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

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
Name: | us-gaap_SiteContingencyLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

X
- Details
Name: | us-gaap_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: | us-gaap_StatementGeographicalAxis=stpr_NH
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |