Document And Entity Information	12 Months Ended
	Dec. 31, 2022 USD ($) shares
Document Information Line Items
Entity Registrant Name	AEI INCOME & GROWTH FUND XXI LIMITED PARTNERSHIP
Trading Symbol	None
Document Type	10-K
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding | shares	17,076.71
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, 2022
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	FY
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
ICFR Auditor Attestation Flag	false
Document Annual 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
Title of 12(g) Security	Limited Partnership Units
Entity Interactive Data Current	Yes
Document Transition Report	false
Auditor Firm ID	542
Auditor Name	Boulay PLLP
Auditor Location	Minneapolis, Minnesota