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Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
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Dec. 31, 2014 Agreement | Dec. 31, 2013
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 |
Balance of OREO | $ 4.0 us-gaap_OtherRealEstate | $ 2.9 us-gaap_OtherRealEstate
Number of interest rate swap agreements | 33 nbtb_NumberOfInterestRateSwapAgreements |
Standby letters of credit expiration period | 1 year |
Mortgage loans on real estate, by loan disclosure | The Company originates and services residential mortgage loans for consumers and sells 15-year, 20-year and 30-year residential real estate mortgages in the secondary market when the interest rate environment is determined to be favorable by management, while retaining servicing rights on the sold loans. |

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- Definition The minimum of days past due before a loan is considered nonperforming. + References No definition available. + Details
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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 Represents expiration period of standby letters of credit. + References No definition available. + Details
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- Details
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- Definition Carrying amount of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. This does not include real estate assets taken in settlement of troubled loans through surrender or foreclosure. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.10(4)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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