Allowance for Loan Losses and Credit Quality of Loans (Details) $ in Thousands	6 Months Ended
Jun. 30, 2019 USD ($) Segment
Allowance for Loan Losses [Abstract]
Loan portfolio segments \| Segment	3
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] \| 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	95.00%
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 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%

No definition available.

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

- 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

- 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

- 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

- 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

- 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

- 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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nbtb_HomeEquityFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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