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Summary of Significant Accounting Policies (Details) | 12 Months Ended
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Dec. 31, 2017
Summary of Significant Accounting Policies [Abstract] |
Minimum number of days past due for nonaccrual loan status | 90 days
Period of sustained repayment performance for nonperforming TDRs to be returned to performing status | 6 months
Standby letters of credit expiration period | 1 year

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- Definition Period of sustained repayment performance for nonperforming TDRs to be returned to performing status, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Represents expiration period of standby letters of credit, 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_StandbyLettersOfCreditExpirationPeriod
Namespace Prefix: | nbtb_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition The threshold period past due for a loan to be considered nonperforming, 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_ThresholdPeriodPastDueForLoansToBeConsideredNonperforming
Namespace Prefix: | nbtb_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_AccountingPoliciesAbstract
Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration