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Contracts and Agreements (Brokerage Commissions) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items] |  |  |
Brokerage commissions | $ 4,643 us-gaap_FloorBrokerageExchangeAndClearanceFees | $ 9,848 us-gaap_FloorBrokerageExchangeAndClearanceFees | $ 21,804 us-gaap_FloorBrokerageExchangeAndClearanceFees
Brokerage commissions as a percentage of average daily net assets | 0.60%usl_PercentageOfAverageDailyNetAssets |  |
Rebalancing Investments Transaction [Member] |  |  |
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items] |  |  |
Brokerage commissions | 3,665 us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember | 4,473 us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember | 8,385 us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember
Brokerage commissions as a percentage of average daily net assets | 78.94%usl_PercentageOfAverageDailyNetAssets / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember | 45.42%usl_PercentageOfAverageDailyNetAssets / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember | 38.46%usl_PercentageOfAverageDailyNetAssets / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember
Trades Necessitated By Creation And Redemption Activity [Member] |  |  |
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items] |  |  |
Brokerage commissions | $ 978 us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember | $ 5,375 us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember | $ 13,419 us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember
Brokerage commissions as a percentage of average daily net assets | 21.06%usl_PercentageOfAverageDailyNetAssets / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember | 54.58%usl_PercentageOfAverageDailyNetAssets / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember | 61.54%usl_PercentageOfAverageDailyNetAssets / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember

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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
Name: | us-gaap_FloorBrokerageExchangeAndClearanceFees
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_LongDurationContractsAssumptionsByProductAndGuaranteeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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- Details
Name: | us-gaap_NatureOfExpenseAxis=usl_RebalancingInvestmentsTransactionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |