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Contracts and Agreements (Brokerage Commissions) (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Long Duration Contracts Assumptions By Product And Guarantee [Line Items]
Brokerage commissions	$ 1,116	$ 2,289	$ 2,516	$ 6,271
Brokerage commissions as a percentage of average daily net assets	0.60%		0.60%
Rebalancing Investments Transaction [Member]
Long Duration Contracts Assumptions By Product And Guarantee [Line Items]
Brokerage commissions			1,984	2,449
Brokerage commissions as a percentage of average daily net assets	78.86%	39.05%	78.86%	39.05%
Trades Necessitated By Creation And Redemption Activity [Member]
Long Duration Contracts Assumptions By Product And Guarantee [Line Items]
Brokerage commissions			$ 532	$ 3,822
Brokerage commissions as a percentage of average daily net assets	21.14%	60.95%	21.14%	60.95%

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