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Document And Entity Information
 3 Months Ended
Mar. 31, 2015
Document Information [Line Items]  
Document Type 10-Q
Amendment Flag false
Document Period End Date Mar. 31, 2015
Document Fiscal Year Focus 2015
Document Fiscal Period Focus Q1
Entity Registrant Name United States 12 Month Natural Gas Fund, LP
Entity Central Index Key 0001405513
Current Fiscal Year End Date --12-31
Entity Filer Category Non-accelerated Filer
Trading Symbol UNL
Entity Common Stock, Shares Outstanding 0dei_EntityCommonStockSharesOutstanding