v3.26.1
Party-in-Interest Transactions (Details) - NBT Bancorp Inc. 401(k) and Employee Stock Ownership Plan [Member] - USD ($)
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Party-in-Interest Transactions [Abstract]    
Total investments at fair value $ 411,209,167 $ 360,983,991
Notes receivable from participants 3,492,310 3,450,393
Total dividend income 20,873,491 12,838,626
Company Common Stock [Member]    
Party-in-Interest Transactions [Abstract]    
Purchases of Company Stock 3,824,520 4,560,353
Sales of Company stock $ 3,236,945 $ 6,099,149
Number of shares of Company stock held in participants accounts (in Shares) 724,330 737,569
Total investments at fair value $ 30,074,182 $ 35,226,305
Total dividend income $ 1,043,188 $ 1,022,466