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Commitments and Contingent Liabilities (Details) (USD $) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 24, 2014
Commitments and Contingent Liabilities [Abstract] |  |  |
Percentage of the Company's loans secured by real estate (in hundredths) | 59.00%nbtb_PercentageOfCompanysLoansSecuredByRealEstate |  |
Guarantor Obligations [Line Items] |  |  |
Loans serviced by the Company for unrelated third parties | $ 626,000,000 nbtb_LoansServicedByCompanyForUnrelatedThirdParties | $ 685,600,000 nbtb_LoansServicedByCompanyForUnrelatedThirdParties |
Mortgage servicing rights | 1,500,000 us-gaap_ServicingAsset | 2,200,000 us-gaap_ServicingAsset |
Federal Reserve Bank Requirement [Abstract] |  |  |
Federal Reserve Bank maintenance period | 14 days |  |
Average reserve at Federal Reserve Bank for maintenance period |  |  | 46,900,000 nbtb_AverageReserveBalancesWithFederalReserveBank
Unused lines of Credit [Member] |  |  |
Guarantor Obligations [Line Items] |  |  |
Commitments - maximum potential obligation | 214,175,000 us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_UnusedLinesOfCreditMember | 216,658,000 us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_UnusedLinesOfCreditMember |
Commitment to Extend Credits [Member] |  |  |
Guarantor Obligations [Line Items] |  |  |
Commitments - maximum potential obligation | 972,912,000 us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = nbtb_CommitmentToExtendCreditsCreditMember | 849,092,000 us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = nbtb_CommitmentToExtendCreditsCreditMember |
Standby Letters of Credit [Member] |  |  |
Guarantor Obligations [Line Items] |  |  |
Commitments - maximum potential obligation | 35,244,000 us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember | 36,837,000 us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember |
Obligation instrument term |  | 5 years |
Commercial Letters of Credit [Member] |  |  |
Guarantor Obligations [Line Items] |  |  |
Commitments - maximum potential obligation | 22,486,000 us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_LetterOfCreditMember | 41,263,000 us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_LetterOfCreditMember |
Loans Sold with Recourse [Member] |  |  |
Guarantor Obligations [Line Items] |  |  |
Commitments - maximum potential obligation | $ 23,351,000 us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = nbtb_LoansSoldWithRecourseMember | $ 15,741,000 us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = nbtb_LoansSoldWithRecourseMember |

X
- Definition For banks and other depository institutions, the calculated average reserve balance outstanding for the specific period as of the Balance Sheet date. Includes noninterest bearing balances with the Federal Reserve Banks. + References No definition available. + Details
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- Definition The dollar amount of principal for mortgage loans being serviced for others. + References No definition available. + Details
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- Definition The percentage of the Company's loans secured by real estate. + References No definition available. + Details
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- Definition Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248 + Details
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- Definition Aggregate amount of servicing assets that are subsequently measured at fair value and servicing assets that are subsequently measured using the amortization method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745 + Details
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