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Allowance for Loan Losses and Credit Quality of Loans, Troubled Debt Restructurings (Details) (USD $) | 3 Months Ended
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Mar. 31, 2015 Contract | Mar. 31, 2014 Contract
Financing Receivable, Modifications [Line Items] |  |
Period of sustained repayment performance for nonperforming TDRs to be returned to performing status | 6 months |
Home Equity [Member] |  |
Financing Receivable, Modifications [Line Items] |  |
Number of contracts | 5 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_HomeEquityMember | 22 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_HomeEquityMember
Pre-modification outstanding recorded investment | $ 0.1 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_HomeEquityMember | $ 1,100,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_HomeEquityMember
Number of TDR loans with subsequent default | 1 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_HomeEquityMember | 1 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_HomeEquityMember
Outstanding recorded investment on TDR loans with subsequent default | 8,000 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_HomeEquityMember | 11,000 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_HomeEquityMember
Direct [Member] |  |
Financing Receivable, Modifications [Line Items] |  |
Number of contracts | 15 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nbtb_DirectFinancingReceivableMember |
Pre-modification outstanding recorded investment | 1,200,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nbtb_DirectFinancingReceivableMember |
Number of TDR loans with subsequent default | 4 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nbtb_DirectFinancingReceivableMember |
Outstanding recorded investment on TDR loans with subsequent default | 200,000 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nbtb_DirectFinancingReceivableMember |
Residential Real Estate Mortgages [Member] |  |
Financing Receivable, Modifications [Line Items] |  |
Number of contracts | 9 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_ResidentialPortfolioSegmentMember | 12 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_ResidentialPortfolioSegmentMember
Pre-modification outstanding recorded investment | 800,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_ResidentialPortfolioSegmentMember | 1,000,000 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_ResidentialPortfolioSegmentMember
Number of TDR loans with subsequent default |  | 1 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_ResidentialPortfolioSegmentMember
Outstanding recorded investment on TDR loans with subsequent default |  | $ 100,000 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_ResidentialPortfolioSegmentMember

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- Definition Period of sustained repayment performance for nonperforming TDRs to be returned to performing status. + References No definition available. + Details
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- Definition Number of financing receivables that have been modified by troubled debt restructurings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524 + Details
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- Definition Pre-modification amount of investment in financing receivables modified by troubled debt restructurings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524 + Details
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- Definition Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953682-111524 + Details
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- Definition Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953682-111524 + Details
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