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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2020
Feb. 16, 2021
Jun. 30, 2020
Document and Entity Information [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2020    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
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    
Entity Public Float     $ 4,548,000
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
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   950,000  
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false