XML 16 R1.htm IDEA: XBRL DOCUMENT v3.25.4
Document and Entity Information - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Feb. 28, 2026
Jun. 30, 2025
Document and Entity Information      
Document Type 10-K    
Document Annual Report true    
Document Transition Report false    
Document Period End Date Dec. 31, 2025    
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 Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
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    
ICFR Auditor Attestation Flag false    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Public Float     $ 603.1
Entity Common Stock, Shares Outstanding   45,604,774  
Documents Incorporated by Reference [Text Block]

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1.Portions of the Proxy Statement for the 2026 Annual Meeting of Stockholders (Part III)

    
Entity Central Index Key 0001979330    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Amendment Flag false    
Auditor Name Elliott Davis, LLC    
Auditor Location Columbia, South Carolina    
Auditor Firm ID 149