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Document And Entity Information
 3 Months Ended
Mar. 31, 2014
Document Information [Line Items]  
Document Type 10-Q
Amendment Flag false
Document Period End Date Mar. 31, 2014
Document Fiscal Year Focus 2014
Document Fiscal Period Focus Q1
Trading Symbol USL
Entity Common Stock, Shares Outstanding 0
Entity Registrant Name United States 12 Month Oil Fund, LP
Entity Central Index Key 0001405528
Current Fiscal Year End Date --12-31
Entity Filer Category Accelerated Filer