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Allowance for Credit Losses and Credit Quality of Loans, Financing Receivables with Payment Default that Were Modified to Borrowers Experiencing Financial Difficulty (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Financing Receivable Modified to Borrowers Experiencing Financial Difficulty [Abstract]
Financing receivables with payment default that were modified to borrowers experiencing financial difficulty	$ 0
Interest Rate Reduction [Member] | Residential [Member]
Financing Receivable Modified to Borrowers Experiencing Financial Difficulty [Abstract]
Financing receivables with payment default that were modified to borrowers experiencing financial difficulty		$ 31
Term Extension [Member] | Residential [Member]
Financing Receivable Modified to Borrowers Experiencing Financial Difficulty [Abstract]
Financing receivables with payment default that were modified to borrowers experiencing financial difficulty		$ 124

X

- Definition

Amortized cost of financing receivable with payment default in current period and modified within previous 12 months preceding payment default for debtor experiencing financial difficulty.

+ References

No definition available.

+ Details

Name:	nbtb_FinancingReceivableModifiedSubsequentDefault
Namespace Prefix:	nbtb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- References

No definition available.

+ Details

Name:	nbtb_FinancingReceivableModifiedToBorrowersExperiencingFinancialDifficultyAbstract
Namespace Prefix:	nbtb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ContractualInterestRateReductionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: