Document And Entity Information	6 Months Ended
	Jun. 30, 2016 USD ($) shares
Document and Entity Information [Abstract]
Entity Registrant Name	AEI Income & Growth Fund XXI Ltd Partnership
Document Type	10-Q
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding | shares	19,841
Entity Public Float | $	$ 0
Amendment Flag	false
Entity Central Index Key	0000931755
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Smaller Reporting Company
Entity Well-known Seasoned Issuer	No
Document Period End Date	Jun. 30, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q2