Fees Paid by Fund and Related Party Transactions (Additional Information) (Details) (USD $)	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Fees Paid and Related Party Transactions [Line Items]
USCF Management Fee	US12OF 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%
Ongoing Registration Fees and Other Offering Expenses	$ 29,362	$ 2,366
Licensing fee incurred	3,844	6,397
Estimated investor tax reporting cost	200,000
US12OF and the Related Public Funds [Member]
Fees Paid and Related Party Transactions [Line Items]
Estimated directors' fees and expenses	560,625
Licensing Agreements [Member] \| First $1,000,000,000 of combined net assets of the funds [Member]
Fees Paid and Related Party Transactions [Line Items]
Fee percentage	0.04%
Licensing Agreements [Member] \| First $1,000,000,000 of combined net assets of the funds [Member] \| Maximum [Member]
Fees Paid and Related Party Transactions [Line Items]
Combined assets basis for determining fee percentage	1,000,000,000
Licensing Agreements [Member] \| Combined net assets above $1,000,000,000 [Member]
Fees Paid and Related Party Transactions [Line Items]
Fee percentage	0.02%
Licensing Agreements [Member] \| Combined net assets above $1,000,000,000 [Member] \| Minimum [Member]
Fees Paid and Related Party Transactions [Line Items]
Combined assets basis for determining fee percentage	$ 1,000,000,000
Licensing Agreements [Member] \| On and after October 20, 2011 [Member]
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.

Name:	us-gaap_LicenseCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 57

-Paragraph 2

-Subparagraph a, b, c, d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- 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

Name:	us-gaap_NoninterestExpenseDirectorsFees
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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=6880815&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

Name:	us-gaap_OtherGeneralExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Base Amount For Fee Percentage

+ References

No definition available.

+ Details

Name:	usl_BaseAmountForFeePercentage
Namespace Prefix:	usl_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Base Fee Percentage

+ References

No definition available.

+ Details

Name:	usl_BaseFeePercentage
Namespace Prefix:	usl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Basis Amount for Fee Percentage

+ References

No definition available.

+ Details

Name:	usl_BasisAmountForFeePercentage
Namespace Prefix:	usl_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	usl_FeesPaidAndRelatedPartyTransactionsLineItems
Namespace Prefix:	usl_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of Average Daily Net Assets

+ References

No definition available.

+ Details

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

- Definition

Registration Fees and Other Offering Expenses

+ References

No definition available.

+ Details

Name:	usl_RegistrationFeesAndOtherOfferingExpenses
Namespace Prefix:	usl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration