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Contracts and Agreements (Brokerage Commissions) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Long-Duration Contracts, Assumptions by Product and Guarantee [Line Items]
Brokerage commissions	$ 4,643us-gaap_FloorBrokerageExchangeAndClearanceFees	$ 9,848us-gaap_FloorBrokerageExchangeAndClearanceFees	$ 21,804us-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	3,665us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember	4,473us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember	8,385us-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	$ 978us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember	$ 5,375us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember	$ 13,419us-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

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

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

X

- Details

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