Allowance for Loan Losses and Credit Quality of Loans, Troubled Debt Restructurings (Details) $ in Millions	3 Months Ended
Mar. 31, 2016 USD ($) Contract	Mar. 31, 2015 USD ($) 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 \| Contract	12	20
Pre-modification outstanding recorded investment \| $	$ 1.0	$ 1.3
Number of TDR loans with subsequent default \| Contract		5
Outstanding recorded investment on TDR loans with subsequent default \| $		$ 0.4
Residential Real Estate Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts \| Contract	4	9
Pre-modification outstanding recorded investment \| $	$ 0.5	$ 0.8
Number of TDR loans with subsequent default \| Contract	1
Outstanding recorded investment on TDR loans with subsequent default \| $	$ 0.2

Period of sustained repayment performance for nonperforming TDRs to be returned to performing status.

No definition available.

Name:	nbtb_PeriodOfSustainedRepaymentPerformanceForNonperformingTdrsToBeReturnedToPerformingStatus
Namespace Prefix:	nbtb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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=49124121&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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=49124121&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- 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

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nbtb_HomeEquityFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: