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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 | 3 Months Ended | 9 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Financing Receivable Modified to Borrowers Experiencing Financial Difficulty [Abstract] |  |  |  |
Financing receivables with payment default that were modified to borrowers experiencing financial difficulty | $ 0 | $ 0 |  |
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 |  |  | $ 59 | $ 171

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- 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

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

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