v3.26.1
Document and Entity Information - shares
6 Months Ended
Jun. 30, 2026
Aug. 05, 2026
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Central Index Key 0001297107  
Current Fiscal Year End Date --12-31  
Document Quarterly Report true  
Document Transition Report false  
Document Period End Date Jun. 30, 2026  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q2  
Entity File Number 001-42730  
Entity Registrant Name COASTALSOUTH BANCSHARES, INC.  
Entity Incorporation, State or Country Code GA  
Entity Tax Identification Number 57-1184730  
Entity Address, Postal Zip Code 30339  
Entity Address, Address Line One 400 Galleria Parkway  
Entity Address, Address Line Two Suite 1900  
Entity Address, City or Town Atlanta  
Entity Address, State or Province GA  
City Area Code 678  
Local Phone Number 396-4605  
Title of 12(b) Security Common Stock, par value $1.00 per share  
Trading Symbol COSO  
Security Exchange Name NYSE  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Emerging Growth Company true  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Shell Company false  
Entity Ex Transition Period false  
Entity Common Stock, Shares Outstanding   11,954,446