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Document and Entity Information	3 Months Ended
	Mar. 31, 2013
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2013
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q1
Trading Symbol	UNL
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
Entity Common Stock Shares Outstanding	0

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