| Document and Entity Information | Oct. 28, 2019 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 28, 2019 |
| Entity Registrant Name | NBT BANCORP INC |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 000-14703 |
| Entity Tax Identification Number | 16-1268674 |
| Entity Address, Address Line One | 52 South Broad Street |
| Entity Address, City or Town | Norwich |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 13815 |
| City Area Code | 607 |
| Local Phone Number | 337-2265 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000790359 |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | NBTB |
| Security Exchange Name | NASDAQ |
