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Segment Information - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2016 Segment Entity Pipeline mi
Segment Reporting Information [Line Items] |
No of segments | Segment | 3
No of subsidiaries | Entity | 3
Length Of Pipeline | 86
Granite State Gas Transmission Inc |
Segment Reporting Information [Line Items] |
Length Of Pipeline | 86
No of major pipeline | Pipeline | 3

X
- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
Name: | us-gaap_NumberOfOperatingSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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

X
- Definition Length of natural gas transmission pipeline. + References No definition available. + Details
Name: | utl_LengthOfPipeline
Namespace Prefix: | utl_
Data Type: | num:lengthItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of Interconnected Interstate Pipelines + References No definition available. + Details
Name: | utl_NumberOfInterconnectedInterstatePipelines
Namespace Prefix: | utl_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of Subsidiaries. + References No definition available. + Details
Name: | utl_NumberOfSubsidiaries
Namespace Prefix: | utl_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=utl_GraniteStateGasTransmissionIncMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |