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Description of Business (Details) - Location |  | 12 Months Ended
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May 02, 2025 | Dec. 31, 2024
Evans Bancorp, Inc [Member] |  |
Acquisition of Evans Bancorp, Inc [Abstract] |  |
Number of banking locations operated by acquiree | 18 | 18

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- Definition Number of banking locations in which the bank operates. + References No definition available. + Details
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Namespace Prefix: | nbtb_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |