Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2012	Mar. 21, 2013	Jun. 30, 2012
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2012
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	FY
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 Well-Known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Common Stock Shares Outstanding		2,350,000
Entity Public Float			$ 40,984,000