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Acquisitions (Details) (USD $) In Millions, unless otherwise specified | 0 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 08, 2013 Branch | Dec. 31, 2014 | Dec. 31, 2013
Alliance Financial Corporation [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Date of acquisition |  | Mar. 08, 2013 |
Total consideration paid | $ 226 us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_BusinessAcquisitionAxis = nbtb_AllianceFinancialCorporationMember |  |
Number of banking centers operated by acquiree | 26 nbtb_NumberOfBankingCentersOperatedByAcquiree / us-gaap_BusinessAcquisitionAxis = nbtb_AllianceFinancialCorporationMember |  |
Other goodwill adjustments [Abstract] |  |  |
Deferred tax adjustments |  | (1.0)us-gaap_IncomeTaxExpenseBenefitContinuingOperationsAdjustmentOfDeferredTaxAssetLiability / us-gaap_BusinessAcquisitionAxis = nbtb_AllianceFinancialCorporationMember |
Valuation adjustments |  | (0.4)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount / us-gaap_BusinessAcquisitionAxis = nbtb_AllianceFinancialCorporationMember |
Decrease in goodwill |  | (1.4)us-gaap_GoodwillPeriodIncreaseDecrease / us-gaap_BusinessAcquisitionAxis = nbtb_AllianceFinancialCorporationMember |
Hampshire First Bank [Member] |  |  |
Other goodwill adjustments [Abstract] |  |  |
Decrease in goodwill |  |  | (1.0)us-gaap_GoodwillPeriodIncreaseDecrease / us-gaap_BusinessAcquisitionAxis = nbtb_HampshireFirstBankMember
Acquisition of Financial Services Company [Member] |  |  |
Other goodwill adjustments [Abstract] |  |  |
Goodwill adjustment related to business acquisition |  |  | $ 0.1 us-gaap_GoodwillPurchaseAccountingAdjustments / us-gaap_BusinessAcquisitionAxis = nbtb_AcquisitionOfFinancialServicesCompanyMember

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- Definition Date when the acquirer obtains control of the acquiree, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e845-128460 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Acquisition Date -URI http://asc.fasb.org/extlink&oid=6919109 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e848-128460 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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