Allowance for Loan Losses and Credit Quality of Loans, Allowance for Loan Losses (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Changes in allowance for loan losses by portfolio segment [Roll Forward]
Balance, beginning of period	$ 69,434,000	$ 69,334,000	$ 71,334,000
Charge-offs	(27,473,000)	(27,689,000)	(26,504,000)
Recoveries	4,859,000	5,365,000	4,235,000
Provision	19,539,000	22,424,000	20,269,000
Balance, end of period	66,359,000	69,434,000	69,334,000
Allowance for loan losses and recorded investment by portfolio segment [Abstract]
Allowance for loan losses	66,359,000	69,434,000	69,334,000
Allowance for loans individually evaluated for impairment	1,100,000	715,000
Allowance for loans collectively evaluated for impairment	65,259,000	68,719,000
Ending balance of loans	5,595,271,000	5,406,795,000
Commercial Loans [Member]
Changes in allowance for loan losses by portfolio segment [Roll Forward]
Balance, beginning of period	35,090,000	35,624,000	38,831,000
Charge-offs	(9,414,000)	(10,459,000)	(8,750,000)
Recoveries	1,774,000	1,957,000	1,641,000
Provision	4,983,000	7,968,000	3,902,000
Balance, end of period	32,433,000	35,090,000	35,624,000
Allowance for loan losses and recorded investment by portfolio segment [Abstract]
Allowance for loan losses	32,433,000	35,090,000	35,624,000
Allowance for loans individually evaluated for impairment	1,100,000	715,000
Allowance for loans collectively evaluated for impairment	31,333,000	34,375,000
Ending balance of loans	2,473,702,000	2,392,621,000
Consumer Loans [Member]
Changes in allowance for loan losses by portfolio segment [Roll Forward]
Balance, beginning of period	27,694,000	27,162,000	26,049,000
Charge-offs	(16,642,000)	(15,459,000)	(15,848,000)
Recoveries	2,800,000	3,136,000	2,556,000
Provision	12,868,000	12,855,000	14,405,000
Balance, end of period	26,720,000	27,694,000	27,162,000
Allowance for loan losses and recorded investment by portfolio segment [Abstract]
Allowance for loan losses	26,720,000	27,694,000	27,162,000
Allowance for loans individually evaluated for impairment	0	0
Allowance for loans collectively evaluated for impairment	26,720,000	27,694,000
Ending balance of loans	2,005,980,000	1,972,537,000
Residential Real Estate Mortgages [Member]
Changes in allowance for loan losses by portfolio segment [Roll Forward]
Balance, beginning of period	6,520,000	6,252,000	6,249,000
Charge-offs	(1,417,000)	(1,771,000)	(1,906,000)
Recoveries	285,000	272,000	38,000
Provision	1,742,000	1,767,000	1,871,000
Balance, end of period	7,130,000	6,520,000	6,252,000
Allowance for loan losses and recorded investment by portfolio segment [Abstract]
Allowance for loan losses	7,130,000	6,520,000	6,252,000
Allowance for loans individually evaluated for impairment	0	0
Allowance for loans collectively evaluated for impairment	7,130,000	6,520,000
Ending balance of loans	1,115,589,000	1,041,637,000
Unallocated [Member]
Changes in allowance for loan losses by portfolio segment [Roll Forward]
Balance, beginning of period	130,000	296,000	205,000
Charge-offs	0	0	0
Recoveries	0	0	0
Provision	(54,000)	(166,000)	91,000
Balance, end of period	76,000	130,000	296,000
Allowance for loan losses and recorded investment by portfolio segment [Abstract]
Allowance for loan losses	76,000	130,000	296,000
Allowance for loans collectively evaluated for impairment	76,000	130,000
Originated Loans [Member]
Allowance for loan losses and recorded investment by portfolio segment [Abstract]
Ending balance of loans	4,847,937,000	4,476,403,000
Ending balance of loans individually evaluated for impairment	20,121,000	21,380,000
Ending balance of loans collectively evaluated for impairment	4,827,816,000	4,455,023,000
Originated Loans [Member] \| Commercial Loans [Member]
Allowance for loan losses and recorded investment by portfolio segment [Abstract]
Ending balance of loans	2,140,371,000	1,990,232,000
Ending balance of loans individually evaluated for impairment	11,079,000	16,120,000
Ending balance of loans collectively evaluated for impairment	2,129,292,000	1,974,112,000
Originated Loans [Member] \| Consumer Loans [Member]
Allowance for loan losses and recorded investment by portfolio segment [Abstract]
Ending balance of loans	1,858,724,000	1,752,950,000
Ending balance of loans individually evaluated for impairment	5,498,000	3,248,000
Ending balance of loans collectively evaluated for impairment	1,853,226,000	1,749,702,000
Originated Loans [Member] \| Residential Real Estate Mortgages [Member]
Allowance for loan losses and recorded investment by portfolio segment [Abstract]
Ending balance of loans	848,842,000	733,221,000
Ending balance of loans individually evaluated for impairment	3,544,000	2,012,000
Ending balance of loans collectively evaluated for impairment	845,298,000	731,209,000
Acquired Loans [Member]
Changes in allowance for loan losses by portfolio segment [Roll Forward]
Balance, beginning of period	0
Charge-offs	(4,800,000)	(400,000)
Balance, end of period	500	0
Allowance for loan losses and recorded investment by portfolio segment [Abstract]
Allowance for loan losses	500	0
Ending balance of loans	747,334,000	930,392,000
Ending balance of loans individually evaluated for impairment	5,675,000	10,060,000
Ending balance of loans collectively evaluated for impairment	741,659,000	920,332,000
Acquired Loans [Member] \| Commercial Loans [Member]
Allowance for loan losses and recorded investment by portfolio segment [Abstract]
Ending balance of loans	333,331,000	402,389,000
Ending balance of loans individually evaluated for impairment	5,675,000	10,060,000
Ending balance of loans collectively evaluated for impairment	327,656,000	392,329,000
Acquired Loans [Member] \| Consumer Loans [Member]
Allowance for loan losses and recorded investment by portfolio segment [Abstract]
Ending balance of loans	147,256,000	219,587,000
Ending balance of loans individually evaluated for impairment	0	0
Ending balance of loans collectively evaluated for impairment	147,256,000	219,587,000
Acquired Loans [Member] \| Residential Real Estate Mortgages [Member]
Allowance for loan losses and recorded investment by portfolio segment [Abstract]
Ending balance of loans	266,747,000	308,416,000
Ending balance of loans individually evaluated for impairment	0	0
Ending balance of loans collectively evaluated for impairment	$ 266,747,000	$ 308,416,000

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

A valuation allowance for financing receivables that are expected to be uncollectible.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of recovery of financing receivables doubtful of collection that were previously charged off.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(4)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Definition

Amount of direct write-downs of financing receivables charged against the allowance.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (h)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (h)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5074-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 11

-Article 9

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.11)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Details

Name:	nbtb_ClassOfFinancingReceivablesAxis=nbtb_AcquiredLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	nbtb_ClassOfFinancingReceivablesAxis=nbtb_OriginatedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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