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Short-Term Borrowings (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Short-term Debt [Line Items] |  |  |
Balance at year-end | $ 316,802,000 us-gaap_ShortTermBorrowings | $ 456,042,000 us-gaap_ShortTermBorrowings |
FHLB, unused lines of credit available for short-term financing | 958,000,000 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity | 497,000,000 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity |
Federal Funds Purchased [Member] |  |  |
Short-term Debt [Line Items] |  |  |
Balance at year-end | 68,000,000 us-gaap_ShortTermBorrowings / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember | 130,000,000 us-gaap_ShortTermBorrowings / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember | 10,000,000 us-gaap_ShortTermBorrowings / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember
Average during the year | 110,154,000 us-gaap_ShorttermDebtAverageOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember | 39,907,000 us-gaap_ShorttermDebtAverageOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember | 12,658,000 us-gaap_ShorttermDebtAverageOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember
Maximum month end balance | 183,000,000 us-gaap_ShorttermDebtMaximumMonthendOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember | 130,000,000 us-gaap_ShorttermDebtMaximumMonthendOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember | 60,000,000 us-gaap_ShorttermDebtMaximumMonthendOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember
Weighted average rate during the year (in hundredths) | 0.29%us-gaap_DebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember | 0.26%us-gaap_DebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember | 0.27%us-gaap_DebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember
Weighted average rate at December 31 (in hundredths) | 0.34%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember | 0.18%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember | 0.27%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = us-gaap_FederalFundsPurchasedMember
Securities Sold under Repurchase Agreements [Member] |  |  |
Short-term Debt [Line Items] |  |  |
Balance at year-end | 148,802,000 us-gaap_ShortTermBorrowings / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 176,042,000 us-gaap_ShortTermBorrowings / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 152,941,000 us-gaap_ShortTermBorrowings / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
Average during the year | 165,858,000 us-gaap_ShorttermDebtAverageOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 169,352,000 us-gaap_ShorttermDebtAverageOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 153,084,000 us-gaap_ShorttermDebtAverageOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
Maximum month end balance | 182,861,000 us-gaap_ShorttermDebtMaximumMonthendOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 185,871,000 us-gaap_ShorttermDebtMaximumMonthendOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 165,977,000 us-gaap_ShorttermDebtMaximumMonthendOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
Weighted average rate during the year (in hundredths) | 0.06%us-gaap_DebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 0.06%us-gaap_DebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 0.10%us-gaap_DebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
Weighted average rate at December 31 (in hundredths) | 0.06%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 0.05%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 0.10%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
Other Short-Term Borrowings [Member] |  |  |
Short-term Debt [Line Items] |  |  |
Balance at year-end | 100,000,000 us-gaap_ShortTermBorrowings / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember | 150,000,000 us-gaap_ShortTermBorrowings / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember | 0 us-gaap_ShortTermBorrowings / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember
Average during the year | 106,438,000 us-gaap_ShorttermDebtAverageOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember | 71,589,000 us-gaap_ShorttermDebtAverageOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember | 0 us-gaap_ShorttermDebtAverageOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember
Maximum month end balance | $ 320,000,000 us-gaap_ShorttermDebtMaximumMonthendOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember | $ 210,000,000 us-gaap_ShorttermDebtMaximumMonthendOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember | $ 0 us-gaap_ShorttermDebtMaximumMonthendOutstandingAmount / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember
Weighted average rate during the year (in hundredths) | 0.40%us-gaap_DebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember | 0.43%us-gaap_DebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember | 0.00%us-gaap_DebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember
Weighted average rate at December 31 (in hundredths) | 0.36%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember | 0.55%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember | 0.00%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_ShortTermDebtTypeAxis = nbtb_OtherShortTermBorrowingsMember

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- Definition Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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- Definition Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Subparagraph a -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),16(a)(1)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Subparagraph a(1) -Article 7 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph 2, 3 -Article 9 + Details
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- Definition For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, maximum month-end borrowings during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Industry Guide -Number 3 -Section VII -Subsection 2 + Details
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- Definition Weighted average interest rate of short-term debt outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Subparagraph b -Article 5 + Details
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