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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2025
Mar. 27, 2026
Jun. 30, 2025
Document and Entity Information      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2025    
Document Transition Report false    
Entity File Number 000-56790    
Entity Registrant Name SENECA BANCORP, INC.    
Entity Incorporation, State or Country Code MD    
Entity Tax Identification Number 39-4029114    
City Area Code 315    
Local Phone Number 638-0233    
Entity Address, Address Line One 35 Oswego Street    
Entity Address, City or Town Baldwinsville    
Entity Address State Or Province NY    
Entity Address, Postal Zip Code 13027    
Title of 12(g) Security Common Stock, Par Value $0.01    
No Trading Symbol Flag true    
Security Exchange Name NONE    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Non-accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag false    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding   1,800,141  
Entity Public Float     $ 0
Documents Incorporated by Reference [Text Block]

Portions of the Definitive Proxy Statement for the 2026 Annual Meeting of Stockholders are incorporated by reference in Part III of the Annual Report on Form 10-K.

    
Auditor Name Bonadio & Co. LLP    
Auditor Location Syracuse, New York    
Auditor Firm ID 1884    
Entity Central Index Key 0002072421    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Amendment Flag false