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Document And Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2015
Mar. 21, 2016
Jun. 30, 2015
Document Information [Line Items]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2015    
Document Fiscal Year Focus 2015    
Document Fiscal Period Focus FY    
Trading Symbol UNL    
Entity Common Stock, Shares Outstanding   1,350,000  
Entity Registrant Name United States 12 Month Natural Gas Fund, LP    
Entity Central Index Key 0001405513    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Non-accelerated Filer    
Entity Public Float     $ 15,336,000