Document And Entity Information (USD $)	9 Months Ended
	Sep. 30, 2011
Entity Registrant Name	AEI Income & Growth Fund XXI Ltd Partnership
Entity Central Index Key	0000931755
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	Smaller Reporting Company
Entity Public Float	$ 0
Entity Common Stock, Shares Outstanding	22,674
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q3
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2011