Acquisitions, Evans Bancorp, Inc. (Details) - Evans Bancorp, Inc [Member] $ in Millions		9 Months Ended
Sep. 09, 2024 USD ($)	Sep. 30, 2024 Location
Acquisition of Evans Bancorp, Inc [Abstract]
Assets of the acquiree \| $	$ 2,280
Number of banking locations operated by acquiree \| Location		18
Share conversion rights	0.91

Number of common shares of the entity issuable to equity holders of the acquiree for each share of the acquiree to be converted.

No definition available.

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Namespace Prefix:	nbtb_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration