Amount of registration fee expense for issuance of shares or units.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 220
-Section 45
-Paragraph 3
-Subparagraph (m)
-URI http://asc.fasb.org/extlink&oid=118262090&loc=SL114874205-224268

+ Details

Name:	us-gaap_InvestmentCompanyRegistrationExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.14)
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

+ 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.6)
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

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

Fees Paid and Related Party Transactions [Line Items]

+ References

No definition available.

+ Details

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

- Definition

It represent the value of license fees and other offering expenses.

+ References

No definition available.

+ Details

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

- Definition

Percentage of Management Fee In Average Daily Total Net Assets.

+ References

No definition available.

+ Details

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

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=usl_FundTrustsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=usl_GroupThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

FEES PAID BY THE FUND AND RELATED PARTY TRANSACTIONS - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2021
Fees Paid and Related Party Transactions [Line Items]
Percentage of management fee in average daily total net assets			0.60%
Registration fees	$ 37,765	$ 39,650	$ 75,115	$ 39,650
Estimated directors' fees and expenses	13,800	4,780	19,330	7,902
Licensing fee incurred	$ 7,444	$ 8,544	$ 15,289	$ 10,251
Scenario Forecast
Fees Paid and Related Party Transactions [Line Items]
Estimated directors' fees and expenses					$ 48,000
Investor tax reporting cost					244,000
USL and the Related Public Funds, other than USCI, USAG, CPER and USMI \| Scenario Forecast
Fees Paid and Related Party Transactions [Line Items]
Estimated directors' fees and expenses					$ 1,061,000
On and after October 20, 2011
Fees Paid and Related Party Transactions [Line Items]
Fee percentage			0.015%