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Document and Entity Information - shares
 3 Months Ended
Mar. 31, 2021
May 06, 2021
Document and Entity Information    
Document Type 10-Q  
Document Quarterly Report true  
Document Transition Report false  
Document Period End Date Mar. 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. Diablo Boulevard, 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   9,400,000
Entity Central Index Key 0001405528  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2021  
Document Fiscal Period Focus Q1  
Amendment Flag false