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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2021
Feb. 22, 2022
Jun. 30, 2021
Document and Entity Information [Abstract]      
Document Annual Report true    
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2021    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
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    
Entity Public Float     $ 10,180,000
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   1,700,000  
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
ICFR Auditor Attestation Flag true    
Auditor Name Spicer Jeffries LLP    
Auditor Firm ID 349    
Auditor Location Denver, Colorado