Summary of Significant Accounting Policies, Other Financial Instruments (Details)	12 Months Ended
Dec. 31, 2023
Other Financial Instruments [Abstract]
Standby letters of credit expiration period	1 year

Represents expiration period of standby letters of credit, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	nbtb_StandbyLettersOfCreditExpirationPeriod
Namespace Prefix:	nbtb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration