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Document and Entity Information - shares
 9 Months Ended
Sep. 30, 2020
Nov. 02, 2020
Document and Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2020  
Document Fiscal Year Focus 2020  
Document Fiscal Period Focus Q3  
Entity Registrant Name United States 12 Month Natural Gas Fund, LP  
Entity Central Index Key 0001405513  
Current Fiscal Year End Date --12-31  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Trading Symbol UNL  
Title of 12(b) Security Shares of United States 12 Month Natural Gas Fund, LP  
Security Exchange Name NYSE  
Entity Common Stock, Shares Outstanding   750,000
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false