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CONTRACTS AND AGREEMENTS (Brokerage Commissions) (Details) - USD ($) | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Total commissions accrued to brokers | $ 318 | $ 434
Total commissions as annualized percentage of average total net assets | 0.02% | 0.01%
Rebalancing Investments Transaction [Member] |  |
Total commissions accrued to brokers | $ 185 | $ 90
Total commissions as annualized percentage of average total net assets | 58.18% | 20.74%
Creation And Redemption Activity [Member] |  |
Total commissions accrued to brokers | $ 133 | $ 344
Total commissions as annualized percentage of average total net assets | 41.82% | 79.26%

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- Definition Percentage of Average Daily Net Assets + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition The amount of expense during the period for floor brokerage fees paid to other broker-dealers to execute trades on their behalf, stock exchange fees, order flow fees, and clearance fees. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementClassOfStockAxis=unl_CreationAndRedemptionActivityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |