Document And Entity Information	9 Months Ended
	Sep. 30, 2020 shares
Document Information Line Items
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	18,791
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, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
City Area Code	651
Document Quarterly Report	true
Document Transition Report	false
Entity Address, Address Line One	30 East 7th Street, Suite 1300
Entity Address, City or Town	St. Paul
Entity Address, Country	US
Entity Address, Postal Zip Code	55101
Entity File Number	000-29274
Entity Incorporation, State or Country Code	MN
Entity Interactive Data Current	Yes
Entity Tax Identification Number	41-1789725
Local Phone Number	227-7333
No Trading Symbol Flag	true
Security Exchange Name	NONE
Title of 12(g) Security	Limited Partnership Units