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v3.20.2

Summary of Significant Accounting Policies (Details) $ in Millions | 9 Months Ended
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Sep. 30, 2020 USD ($) Loan
Minimum [Member] |
Allowance for Credit Losses [Abstract] |
Threshold period past due for loans to be written off | 90 days
Maximum [Member] |
Allowance for Credit Losses [Abstract] |
Threshold period past due for loans to be written off | 120 days
Paycheck Protection Program [Member] |
Allowance for Credit Losses [Abstract] |
Number of loans processed | Loan | 3,000
Paycheck Protection Program [Member] | Minimum [Member] |
Allowance for Credit Losses [Abstract] |
Amount of loans processed | $ | $ 548
Commercial Loans [Member] | Commercial Real Estate [Member] | Maximum [Member] |
Allowance for Credit Losses [Abstract] |
Loan amount, percentage of appraised value or purchase price of the property | 80.00%
Consumer Loans [Member] | Auto [Member] | Minimum [Member] |
Allowance for Credit Losses [Abstract] |
Principal repayment term of loan | 3 years
Consumer Loans [Member] | Auto [Member] | Maximum [Member] |
Allowance for Credit Losses [Abstract] |
Principal repayment term of loan | 6 years
Consumer Loans [Member] | Installment Loans [Member] | Minimum [Member] |
Allowance for Credit Losses [Abstract] |
Principal repayment term of loan | 1 year
Consumer Loans [Member] | Installment Loans [Member] | Maximum [Member] |
Allowance for Credit Losses [Abstract] |
Principal repayment term of loan | 15 years
Residential [Member] | Maximum [Member] |
Allowance for Credit Losses [Abstract] |
Loan amount, percentage of appraised value or purchase price of the property | 85.00%
Residential [Member] | Home Equity [Member] | Maximum [Member] |
Allowance for Credit Losses [Abstract] |
Loan amount, percentage of equity in property | 85.00%
Term of draw | 10 years
Term of amortization | 15 years

X
- Definition Represents the amortization term of the financing receivable, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | nbtb_FinancingReceivableAmortizationTerm
Namespace Prefix: | nbtb_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of financing receivables processed. + References No definition available. + Details
Name: | nbtb_FinancingReceivableAmountOfLoansProcessed
Namespace Prefix: | nbtb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Number of financing receivables processed. + References No definition available. + Details
Name: | nbtb_FinancingReceivableNumberOfLoansProcessed
Namespace Prefix: | nbtb_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the principal repayment term of the financing receivable, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | nbtb_FinancingReceivablePrincipalRepaymentTerm
Namespace Prefix: | nbtb_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the draw term of the financing receivable, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | nbtb_FinancingReceivableTermOfDraw
Namespace Prefix: | nbtb_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition This element represents the percentage of the appraised value or purchase price of a property for the originated amounts of loans. + References No definition available. + Details
Name: | nbtb_LoanOriginationAmountPercentageOfAppraisedValueOrPurchasePriceOfProperty
Namespace Prefix: | nbtb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition This element represents the percentage of the equity in a property for the originated amounts of loans. + References No definition available. + Details
Name: | nbtb_LoanOriginationAmountPercentageOfEquityInProperty
Namespace Prefix: | nbtb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The threshold period past due for a loan to be written off, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | nbtb_ThresholdPeriodPastDueForLoansToBeWrittenOff
Namespace Prefix: | nbtb_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_AllowanceForCreditLossAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis=nbtb_SmallBusinessAdministrationSBACARESActPaycheckProtectionProgramMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nbtb_AutoFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nbtb_InstallmentFinancingReceivableMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nbtb_HomeEquityFinancingReceivablesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |