v3.26.1
Document and Entity Information - shares
 3 Months Ended
Mar. 31, 2026
Apr. 30, 2026
Document and Entity Information    
Document Type 10-Q  
Document Quarterly Report true  
Document Transition Report false  
Document Period End Date Mar. 31, 2026  
Entity File Number 001-41899  
Entity Registrant Name NB Bancorp, Inc.  
Entity Incorporation, State or Country Code MD  
Entity Tax Identification Number 93-2560883  
Entity Address State Or Province MA  
Entity Address, Address Line One 1063 Great Plain Avenue  
Entity Address, City or Town Needham  
Entity Address, Postal Zip Code 02492  
City Area Code 781  
Local Phone Number 444-2100  
Title of 12(b) Security Common stock, par value $0.01 per share  
Trading Symbol NBBK  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   44,706,163
Entity Central Index Key 0001979330  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q1  
Amendment Flag false