Document And Entity Information	9 Months Ended
	Sep. 30, 2018 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	19,402
Entity Public Float | $	$ 0
Amendment Flag	false
Entity Central Index Key	0000931755
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q3
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Ex Transition Period	false