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Commitments and Contingent Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Commitments and Contingent Liabilities [Abstract]
Percentage of the Company's loans secured by real estate	59.00%
Guarantor Obligations [Abstract]
Obligation instrument term	1 year
Federal Reserve Bank Requirement [Abstract]
Federal Reserve Bank maintenance period	14 days
Average reserve at Federal Reserve Bank for maintenance period	$ 64,600
Unused lines of Credit [Member]
Guarantor Obligations [Abstract]
Commitments - maximum potential obligation	351,876	$ 340,703
Commitment to Extend Credits, Primarily Variable Rate [Member]
Guarantor Obligations [Abstract]
Commitments - maximum potential obligation	1,831,671	1,579,414
Standby Letters of Credit [Member]
Guarantor Obligations [Abstract]
Commitments - maximum potential obligation	54,009	34,479
Loans Sold with Recourse [Member]
Guarantor Obligations [Abstract]
Commitments - maximum potential obligation	$ 25,659	$ 25,639

X

- Definition

For banks and other depository institutions, the calculated average reserve balance outstanding for the specific period as of the Balance Sheet date. Includes noninterest bearing balances with the Federal Reserve Banks.

+ References

No definition available.

+ Details

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Period Type:	instant

X

- References

No definition available.

+ Details

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X

- Definition

Describe the approximate period of the guarantee or each group of similar guarantees, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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X

- Definition

Specific period of time established required to maintain reserve balances with the Federal Reserve Bank, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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X

- Definition

The percentage of the Company's loans secured by real estate.

+ References

No definition available.

+ Details

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Namespace Prefix:	nbtb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
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X

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=121555522&loc=d3e12069-110248

+ Details

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Balance Type:	credit
Period Type:	instant

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- References

No definition available.

+ Details

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- Details

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- Details

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- Details

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- Details

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