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Financial Highlights (Tables) | 3 Months Ended
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Mar. 31, 2014
Per Unit Performance Data and Other Supplemental Financial Data | The following table presents per share performance data and other supplemental financial data for the three months ended March 31, 2014 and 2013 for the shareholders. This information has been derived from information presented in the condensed financial statements.
|  | For the three months ended |  |  | For the three months ended |
|  | March 31, 2014 |  |  | March 31, 2013 |
|  | (Unaudited) |  |  | (Unaudited) |
Per Share Operating Performance: |  |  |  |  |  |  |  |
Net asset value, beginning of period |  | $ | 42.83 |  |  | $ | 39.81 |
Total income |  |  | 1.38 |  |  |  | 1.32 |
Total expenses |  |  | (0.11) |  |  |  | (0.10) |
Net increase in net asset value |  |  | 1.27 |  |  |  | 1.22 |
Net asset value, end of period |  | $ | 44.10 |  |  | $ | 41.03 |
Total Return |  |  | 2.97 | % |  |  | 3.06 | %
Ratios to Average Net Assets |  |  |  |  |  |  |  |
Total income |  |  | 2.80 | % |  |  | 3.36 | %
Expenses excluding management fees* |  |  | 0.41 | % |  |  | 0.36 | %
Management fees* |  |  | 0.60 | % |  |  | 0.60 | %
Net income |  |  | 2.55 | % |  |  | 3.12 | %
* Annualized

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- Definition Per Unit Performance Data and Other Supplemental Financial Data [Table Text Block] + References No definition available. + Details
Name: | usl_PerUnitPerformanceDataAndOtherSupplementalFinancialDataTableTextBlock
Namespace Prefix: | usl_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration