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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Mar. 06, 2024
Jun. 30, 2023
Document and Entity Information        
Document Type 10-K      
Document Period End Date Dec. 31, 2023      
Document Annual Report true      
Document Transition Report false      
Entity File Number 001-34535      
Entity Registrant Name United States 12 Month Natural Gas Fund, LP      
Entity Incorporation, State or Country Code DE      
Entity Tax Identification Number 26-0431733      
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      
Title of 12(b) Security Shares of United States 12 Month Natural Gas Fund, LP      
Trading Symbol UNL      
Security Exchange Name NYSE      
Entity Current Reporting Status Yes      
Entity Interactive Data Current Yes      
Entity Filer Category Non-accelerated Filer      
Entity Small Business true      
Entity Emerging Growth Company false      
Entity Shell Company false      
Entity Common Stock, Shares Outstanding     2,000,000  
Entity Central Index Key 0001405513      
Current Fiscal Year End Date --12-31      
Document Fiscal Year Focus 2023      
Document Fiscal Period Focus FY      
Amendment Flag false      
Entity Public Float       $ 17,270,293
Entity Well-known Seasoned Issuer No      
Entity Voluntary Filers No      
ICFR Auditor Attestation Flag true      
Document Financial Statement Error Correction [Flag] false      
Auditor Name Cohen & Company, Ltd. Spicer Jeffries LLP    
Auditor Firm ID 925 349    
Auditor Location Philadelphia, Pennsylvania Denver, Colorado