v3.26.1
RELATED PARTY AND PARTY-IN-INTEREST TRANSACTIONS (Details) - Needham Bank 401(k) Plan - USD ($)
12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
RELATED PARTY AND PARTY-IN-INTEREST TRANSACTIONS    
Custodian fees $ 81,440  
Custodian    
RELATED PARTY AND PARTY-IN-INTEREST TRANSACTIONS    
Custodian fees $ 81,440  
NB Bancorp, Inc | Common stock    
RELATED PARTY AND PARTY-IN-INTEREST TRANSACTIONS    
Number of shares held 600,002 633,201