Document And Entity Information	12 Months Ended
	Dec. 31, 2018 USD ($) shares
Document and Entity Information [Abstract]
Entity Registrant Name	AEI Income & Growth Fund XXI Ltd Partnership
Document Type	10-K
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding | shares	19,329
Entity Public Float | $	$ 0
Amendment Flag	false
Entity Central Index Key	0000931755
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Well-known Seasoned Issuer	No
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Ex Transition Period	false