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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2014 mi Bcf | Dec. 31, 2013 Bcf | Mar. 31, 2013 Bcf
Significant Accounting Policies [Line Items] |  |  |
Length Of Pipeline | 86 |  |
Cost of removal obligation | $ 59.2 | $ 57.3 | $ 53.0
Number of Natural Gas Storage Outstanding | 1.1 | 1.8 | 1.7
Maximum |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cash equivalents maturity period | 3 months |  |
ISO-NE Obligations |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cash Deposits | 9.8 | 7.3 | 5.6
Natural Gas Hedging Program |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cash Deposits | $ 0.1 | $ 0 | $ 0
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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Period Type: | instant

X
- Definition Cash Deposits + References No definition available. + Details
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- Definition Cash Equivalents, Maturity Period + References No definition available. + Details
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X
- Definition Length of natural gas transmission pipeline. + References No definition available. + Details
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X
- Definition Amount of natural gas purchase contracts outstanding under heging program. + References No definition available. + Details
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Namespace Prefix: | utl_
Data Type: | xbrli:decimalItemType
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X
- Definition Number of Subsidiaries. + References No definition available. + Details
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X
- Details
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Namespace Prefix: | utl_
Data Type: | xbrli:stringItemType
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