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Document and Entity Information - shares
 6 Months Ended
Jun. 30, 2025
Aug. 12, 2025
Document and Entity Information [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2025  
Document Transition Report false  
Entity File Number 001-42198  
Entity Registrant Name Fifth District Bancorp, Inc.  
Entity Incorporation, State or Country Code MD  
Entity Tax Identification Number 99-1897673  
Entity Address, Address Line One 4000 General DeGaulle Drive  
Entity Address, City or Town New Orleans  
Entity Address, State or Province LA  
Entity Address, Postal Zip Code 70114  
City Area Code 504  
Local Phone Number 362-7544  
Title of 12(b) Security Common Stock, $0.01 par value per share  
Trading Symbol FDSB  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   5,559,473
Entity Central Index Key 0002012726  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus Q2  
Amendment Flag false