Document And Entity Information	6 Months Ended
	Jun. 30, 2021 shares
Document Information Line Items
Entity Registrant Name	AEI INCOME & GROWTH FUND XXI LIMITED PARTNERSHIP
Document Type	10-Q
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding	17,947.17
Amendment Flag	false
Entity Central Index Key	0000931755
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Document Period End Date	Jun. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Document Quarterly Report	true
Entity File Number	000-29274
Entity Incorporation, State or Country Code	MN
Entity Tax Identification Number	41-1789725
Entity Address, Address Line One	30 East 7th Street, Suite 1300
Entity Address, City or Town	St. Paul
Entity Address, State or Province	MN
Entity Address, Postal Zip Code	55101
City Area Code	651
Local Phone Number	227-7333
Entity Information, Former Legal or Registered Name	Not Applicable
No Trading Symbol Flag	true
Security Exchange Name	NONE
Title of 12(g) Security	Limited Partnership Units
Entity Interactive Data Current	Yes
Document Transition Report	false