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Commitments and Contingencies (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Commitment to Extend Credits and Unused Lines of Credit [Member]
Guarantor Obligations [Line Items]
Commitments - maximum potential obligation	$ 1,300.0	$ 1,200.0
Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Commitments - maximum potential obligation	$ 32.4	35.2
Obligation instrument term	5 years
Commercial Letters of Credit [Member]
Guarantor Obligations [Line Items]
Commitments - maximum potential obligation	$ 5.3	$ 22.5

X

- Definition

Describe the approximate period of the guarantee or each group of similar guarantees.

+ References

No definition available.

+ Details

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

X

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

X

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Topic 460



-SubTopic 10



-Section 50



-Paragraph 4



-Subparagraph (b)(1)



-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=nbtb_CommitmentToExtendCreditAndUnusedLinesOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_FinancialStandbyLetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_LetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: