Fees Paid by the Fund and Related Party Transactions (Additional Information) (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fees Paid and Related Party Transactions [Line Items]
USCF Management Fee	USL is contractually obligated to pay USCF a fee, which is paid monthly, equal to 0.60%per annum of average daily total net assets.
Percentage of average daily total net assets	0.60%usl_PercentageOfAverageDailyNetAssets
Ongoing Registration Fees and Other Offering Expenses	$ 1,350	$ 82,385	$ 44,844
Estimated directors' fees and expenses	15,649us-gaap_NoninterestExpenseDirectorsFees	19,340us-gaap_NoninterestExpenseDirectorsFees	25,332us-gaap_NoninterestExpenseDirectorsFees
Licensing fee incurred	7,910us-gaap_LicenseCosts	12,774us-gaap_LicenseCosts	20,192us-gaap_LicenseCosts
Estimated investor tax reporting cost	169,000us-gaap_OtherGeneralExpense	150,000us-gaap_OtherGeneralExpense	150,000us-gaap_OtherGeneralExpense
USL and the Related Public Funds, other than USCI, USAG, CPER and USMI
Fees Paid and Related Party Transactions [Line Items]
Estimated directors' fees and expenses	567,863	555,456	540,586
USL
Fees Paid and Related Party Transactions [Line Items]
Estimated directors' fees and expenses	$ 15,649	$ 19,340	$ 25,332
Licensing Agreements \| On and after October 20, 2011
Fees Paid and Related Party Transactions [Line Items]
Fee percentage	0.015%
Assets basis for determining fee percentage	on all net assets

Costs incurred and are directly related to generating license revenue. Licensing arrangements include, but are not limited to, rights to use a patent, copyright, technology, manufacturing process, software or trademark.

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- Definition

Description of the nature of payments to managing member or general partner for management of the day-to-day business functions of the limited liability company (LLC) or limited partnership (LP), including the fee rate, basis of calculation, relevant accounting period, whether the fee is paid to an entity other than the managing member or general partner, or whether the fee is waived.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 850

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

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- Definition

Noninterest expense related to directors' fees which are fees paid by an Entity to its directors. Directors' fees may be paid in addition to salary and other benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.14)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

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- Definition

Amount of general expenses not normally included in Other Operating Costs and Expenses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.6)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 6

-Article 5

+ Details

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- Definition

Base Amount For Fee Percentage

+ References

No definition available.

+ Details

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- Definition

Base Fee Percentage

+ References

No definition available.

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- Definition

Percentage of Average Daily Net Assets

+ References

No definition available.

+ Details

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- Definition

Registration Fees and Other Offering Expenses

+ References

No definition available.

+ Details

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