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Summary of Significant Accounting Policies (Additional Information) (Details)				12 Months Ended
	Dec. 31, 2013	Feb. 29, 2012 Creation Baskets	Feb. 29, 2012 Redemption Baskets	Dec. 31, 2013 Maximum
Significant Accounting Policies [Line Items]
Minimum likelihood of tax benefits being recognized upon ultimate settlement	50.00%
Cash equivalents maturity period				6 months
Number of Shares per basket		50,000	100,000

X

- Definition

Cash Equivalents, Maturity Period

+ References

No definition available.

+ Details

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Namespace Prefix:	usl_
Data Type:	xbrli:durationItemType
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X

- Definition

Number of Shares per Basket.

+ References

No definition available.

+ Details

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Namespace Prefix:	usl_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Percentage Of Income Tax Examination Minimum Likelihood of Tax Benefits Being Realized Upon Settlement

+ 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:	usl_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration