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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2021
Feb. 22, 2022
Jun. 30, 2021
Document and Entity Information      
Document Type 10-K    
Document Annual Report true    
Document Transition Report false    
Document Period End Date Dec. 31, 2021    
Entity File Number 001-33859    
Entity Registrant Name United States 12 Month Oil Fund, LP    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 26-0431897    
Entity Address, Address Line One 1850 Mt    
Entity Address, Address Line Two Diablo Boulevard    
Entity Address, Address Line Three Suite 640    
Entity Address, City or Town Walnut Creek    
Entity Address, State or Province CA    
Entity Address, Postal Zip Code 94596    
City Area Code 510    
Local Phone Number 522-9600    
Title of 12(b) Security Shares of United States 12 Month Oil Fund, LP    
Trading Symbol USL    
Security Exchange Name NYSE    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding   4,550,000  
Entity Central Index Key 0001405528    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Voluntary Filers No    
Entity Well-known Seasoned Issuer No    
Entity Public Float     $ 188,013,000
ICFR Auditor Attestation Flag false    
Auditor Name Spicer Jeffries LLP    
Auditor Firm ID 349    
Auditor Location Denver, Colorado