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Segment Information - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2012 mi Entity Segment
Segment Reporting Information [Line Items]
No of segments	4
No of subsidiaries	3
Length of pipeline	86
Granite State Gas Transmission Inc
Segment Reporting Information [Line Items]
Length of pipeline	86
No of major pipeline	3

X

- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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- Definition

Length Of Pipeline

+ References

No definition available.

+ Details

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Namespace Prefix:	utl_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

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- 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

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- Definition

Number Of Segments

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+ Details

Name:	utl_NumberOfSegments
Namespace Prefix:	utl_
Data Type:	xbrli:integerItemType
Balance Type:	na
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- Definition

Number of Subsidiaries

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No definition available.

+ Details

Name:	utl_NumberOfSubsidiaries
Namespace Prefix:	utl_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration