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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2013 mi Bcf | Dec. 31, 2012 Bcf | Sep. 30, 2012 Bcf
Significant Accounting Policies [Line Items] |  |  |
Length Of Pipeline | 86 |  |
Cost of removal obligation | $ 56.7 | $ 51.4 | $ 50.3
Number of Natural Gas Storage Outstanding | 2,600,000,000 | 1,900,000,000 | 2,100,000,000
Utilities |  |  |
Significant Accounting Policies [Line Items] |  |  |
Number of Subsidiaries | 3 |  |
Other Subsidiaries |  |  |
Significant Accounting Policies [Line Items] |  |  |
Number of Subsidiaries | 3 |  |

X
- Definition Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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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 Natural Gas And Oil Proved Reserves Multiplier To Cover Fair Value Of Transactions Outstanding + References No definition available. + Details
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Namespace Prefix: | utl_
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- Definition Number of Subsidiaries + References No definition available. + Details
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Namespace Prefix: | utl_
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- Details
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Data Type: | xbrli:stringItemType
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