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Document and Entity Information - shares
 3 Months Ended
Mar. 31, 2021
Apr. 26, 2021
Document and Entity Information [Abstract]    
Document Quarterly Report true  
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Mar. 31, 2021  
Document Fiscal Year Focus 2021  
Document Fiscal Period Focus Q1  
Entity Registrant Name United States 12 Month Natural Gas Fund, LP  
Document Transition Report false  
Entity File Number 001-34535  
Entity Central Index Key 0001405513  
Entity Incorporation, State or Country Code DE  
Entity Address, Address Line One 1850 Mt. Diablo Boulevard  
Entity Address, Address Line Two 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  
Entity Tax Identification Number 26-0431733  
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   1,050,000
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false