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Summary of Significant Accounting Policies (Details) $ in Millions	9 Months Ended
	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: