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Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Commitment to Extend Credits and Unused Lines of Credit [Member]
Guarantor Obligations [Line Items]
Commitments - maximum potential obligation	$ 1,200.0us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = nbtb_CommitmentToExtendCreditAndUnusedLinesOfCreditMember	$ 1,200.0us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = nbtb_CommitmentToExtendCreditAndUnusedLinesOfCreditMember
Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Commitments - maximum potential obligation	35.4us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember	35.2us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember
Obligation instrument term	5 years
Commercial Letters of Credit [Member]
Guarantor Obligations [Line Items]
Commitments - maximum potential obligation	$ 10.9us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_LetterOfCreditMember	$ 22.5us-gaap_GuaranteeObligationsMaximumExposure / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_LetterOfCreditMember

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
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X

- Details

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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_
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Balance Type:	credit
Period Type:	instant

X

- Details

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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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