v3.26.1
Document And Entity Information - shares
 6 Months Ended
Jun. 30, 2026
Jul. 31, 2026
Document Information Line Items    
Entity Central Index Key 0000931755  
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2026  
Document Fiscal Year Focus 2026  
Entity File Number 000-29274  
Entity Registrant Name AEI INCOME & GROWTH FUND XXI LIMITED PARTNERSHIP  
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  
Security Exchange Name NONE  
No Trading Symbol Flag true  
Title of 12(g) Security Limited Partnership Units  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company false  
Document Transition Report false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   15,970.84
Amendment Flag false  
Document Fiscal Period Focus Q2  
Current Fiscal Year End Date --12-31