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Summary of Significant Accounting Policies - Additional Information (Detail) | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2012 mi Bcf | Dec. 31, 2011 Bcf | Sep. 30, 2011 Bcf
Significant Accounting Policies [Line Items] |  |  |
Number of Operating Gas Transmission | 86 |  |
Natural Gas Outstanding under purchase contract | 2,100,000,000 | 1,600,000,000 | 1,700,000,000
Other Subsidiaries |  |  |
Significant Accounting Policies [Line Items] |  |  |
Number of Subsidiaries | 3 |  |
Utilities |  |  |
Significant Accounting Policies [Line Items] |  |  |
Number of Subsidiaries | 3 |  |

X
- Definition Length Of Pipeline + References No definition available. + Details
Name: | utl_LengthOfPipeline
Namespace Prefix: | utl_
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Balance Type: | na
Period Type: | duration

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

X
- Details
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Namespace Prefix: | utl_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Volume of Energy Commodity Contracts, Natural Gas + References No definition available. + Details
Name: | utl_VolumeOfEnergyCommodityContractsNaturalGas
Namespace Prefix: | utl_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant