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Allowance for Loan Losses and Credit Quality of Loans (Details) $ in Thousands | 6 Months Ended |
--- | --- | ---
Jun. 30, 2018 USD ($) Segment | Mar. 31, 2018
Allowance for Loan Losses [Abstract] |  |
Loan portfolio segments | Segment | 3 |
Agricultural [Member] | Maximum [Member] |  |
Allowance for Loan Losses [Abstract] |  |
Loan amount, percentage of commercial portfolio |  | 3.00%
Specialty Lending [Member] |  |
Allowance for Loan Losses [Abstract] |  |
Loan amount, percentage of consumer portfolio |  | 21.00%
Commercial Loans [Member] | Commercial Real Estate [Member] | Maximum [Member] |  |
Allowance for Loan Losses [Abstract] |  |
Loan amount, percentage of appraised value or purchase price of the property | 80.00% |
Commercial Loans [Member] | Agricultural Real Estate [Member] | Maximum [Member] |  |
Allowance for Loan Losses [Abstract] |  |
Loan amount, percentage of appraised value or purchase price of the property | 75.00% |
Commercial Loans [Member] | Business Banking [Member] | Maximum [Member] |  |
Allowance for Loan Losses [Abstract] |  |
Business banking loans, amount available | $ | $ 750 |
Consumer Loans [Member] | Dealer Finance [Member] |  |
Allowance for Loan Losses [Abstract] |  |
Percentage of automobile financing to indirect relationships with dealers | 70.00% |
Consumer Loans [Member] | Dealer Finance [Member] | Minimum [Member] |  |
Allowance for Loan Losses [Abstract] |  |
Principal repayment term of loan | 3 years |
Consumer Loans [Member] | Dealer Finance [Member] | Maximum [Member] |  |
Allowance for Loan Losses [Abstract] |  |
Principal repayment term of loan | 6 years |
Consumer Loans [Member] | Direct [Member] | Minimum [Member] |  |
Allowance for Loan Losses [Abstract] |  |
Principal repayment term of loan | 1 year |
Consumer Loans [Member] | Direct [Member] | Maximum [Member] |  |
Allowance for Loan Losses [Abstract] |  |
Principal repayment term of loan | 10 years |
Consumer Loans [Member] | Home Equity [Member] | Maximum [Member] |  |
Allowance for Loan Losses [Abstract] |  |
Loan amount, percentage of appraised value or purchase price of the property | 85.00% |
Loan amount, percentage of equity in property | 85.00% |
Residential Real Estate [Member] | Maximum [Member] |  |
Allowance for Loan Losses [Abstract] |  |
Loan amount, percentage of appraised value or purchase price of the property | 85.00% |

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

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 This element represents the percentage of the loan amount to commercial portfolio. + References No definition available. + Details
Name: | nbtb_LoanAmountPercentageOfCommercialPortfolio
Namespace Prefix: | nbtb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition This element represents the percentage of the loan amount to consumer portfolio. + References No definition available. + Details
Name: | nbtb_LoanAmountPercentageOfConsumerPortfolio
Namespace Prefix: | nbtb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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 This element represents the number of portfolio segments within the loan portfolio. + References No definition available. + Details
Name: | nbtb_LoanPortfolioSegments
Namespace Prefix: | nbtb_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the percentage of automobile financing to indirect relationships the Company maintains with dealers. + References No definition available. + Details
Name: | nbtb_PercentageOfAutomobileFinancingToIndirectRelationshipsWithDealers
Namespace Prefix: | nbtb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition This element represents the loan amount available for business banking customers. + References No definition available. + Details
Name: | nbtb_SmallBusinessLoansAmountAvailable
Namespace Prefix: | nbtb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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

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

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

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

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

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- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nbtb_HomeEquityFinancingReceivablesMember
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: |