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v3.25.2

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 | 6 Months Ended
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Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Financing Receivable Modified to Borrowers Experiencing Financial Difficulty [Abstract] |  |  |  |
Amortized cost basis of modified financing receivables that subsequently defaulted | $ 11 | $ 171 | $ 69 | $ 171
Residential [Member] |  |  |  |
Financing Receivable Modified to Borrowers Experiencing Financial Difficulty [Abstract] |  |  |  |
Amortized cost basis of modified financing receivables that subsequently defaulted | $ 11 | $ 171 | $ 69 | $ 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_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |