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Contracts and Agreements (Brokerage Commissions) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Long Duration Contracts Assumptions By Product And Guarantee [Line Items]
Brokerage commissions	$ 6,437us-gaap_FloorBrokerageExchangeAndClearanceFees	$ 1,400us-gaap_FloorBrokerageExchangeAndClearanceFees
Brokerage commissions as a percentage of average daily net assets	0.04%usl_PercentageOfAverageDailyNetAssets	0.01%usl_PercentageOfAverageDailyNetAssets
Rebalancing Investments Transaction [Member]
Long Duration Contracts Assumptions By Product And Guarantee [Line Items]
Brokerage commissions	4,323us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember	999us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember
Brokerage commissions as a percentage of average daily net assets	67.16%usl_PercentageOfAverageDailyNetAssets / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember	71.36%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	$ 2,114us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember	$ 401us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember
Brokerage commissions as a percentage of average daily net assets	32.84%usl_PercentageOfAverageDailyNetAssets / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember	28.64%usl_PercentageOfAverageDailyNetAssets / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember

X

- 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_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of Average Daily Net Assets

+ References

No definition available.

+ Details

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Namespace Prefix:	usl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: