The threshold balance of financing receivable that is individually evaluated for impairment.

No definition available.

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Period Type:	instant

- Definition

Threshold balance of financing receivables classified as a troubled debt restructuring that are collectively evaluated for impairment.

+ References

No definition available.

+ Details

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Namespace Prefix:	cbu_
Data Type:	xbrli:monetaryItemType
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- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

No definition available.

+ Details

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- 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

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- 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=84173941&loc=SL6953676-111524

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

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

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of the outstanding recorded investment related to 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 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

+ Details

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- Details

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- Details

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LOANS, Troubled Debt Restructuring (TDR) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Loan	Sep. 30, 2016 USD ($) Loan	Sep. 30, 2017 USD ($) Loan	Sep. 30, 2016 USD ($) Loan	Dec. 31, 2016 USD ($) Loan
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	217		217		214
TDRs, amount	$ 6,238		$ 6,238		$ 5,530
Loans modified in TDR during the year, number \| Loan	19	11	48	41
Loans modified in TDR during the year, amount	$ 781	$ 295	$ 1,890	$ 1,307
Maximum [Member]
Financing Receivable, Modifications [Line Items]
Threshold balance of TDR loans collectively included in general loan loss allocation and qualitative review	$ 500		$ 500
Nonaccrual [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	65		65		56
TDRs, amount	$ 2,807		$ 2,807		$ 1,832
Accruing [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	152		152		158
TDRs, amount	$ 3,431		$ 3,431		$ 3,698
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	92		92		81
TDRs, amount	$ 4,071		$ 4,071		$ 3,476
Loans modified in TDR during the year, number \| Loan	8	2	15	9
Loans modified in TDR during the year, amount	$ 540	$ 206	$ 1,040	$ 787
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member] \| Nonaccrual [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	45		45		36
TDRs, amount	$ 2,131		$ 2,131		$ 1,520
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member] \| Accruing [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	47		47		45
TDRs, amount	$ 1,940		$ 1,940		$ 1,956
Commercial Portfolio Segment [Member] \| Minimum [Member]
Financing Receivable, Modifications [Line Items]
Threshold balance of loans individually evaluated for impairment	$ 500		$ 500
Commercial Portfolio Segment [Member] \| Business Lending [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	14		14		11
TDRs, amount	$ 836		$ 836		$ 781
Loans modified in TDR during the year, number \| Loan	1	0	4	1
Loans modified in TDR during the year, amount	$ 51	$ 0	$ 414	$ 29
Commercial Portfolio Segment [Member] \| Business Lending [Member] \| Nonaccrual [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	9		9		6
TDRs, amount	$ 453		$ 453		$ 91
Commercial Portfolio Segment [Member] \| Business Lending [Member] \| Accruing [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	5		5		5
TDRs, amount	$ 383		$ 383		$ 690
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	70		70		78
TDRs, amount	$ 839		$ 839		$ 771
Loans modified in TDR during the year, number \| Loan	8	9	22	27
Loans modified in TDR during the year, amount	$ 181	$ 89	$ 323	$ 392
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member] \| Nonaccrual [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	0		0		0
TDRs, amount	$ 0		$ 0		$ 0
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member] \| Accruing [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	70		70		78
TDRs, amount	$ 839		$ 839		$ 771
Consumer Portfolio Segment [Member] \| Consumer Direct [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	23		23		23
TDRs, amount	$ 62		$ 62		$ 65
Loans modified in TDR during the year, number \| Loan	1	0	4	1
Loans modified in TDR during the year, amount	$ 1	$ 0	$ 7	$ 51
Consumer Portfolio Segment [Member] \| Consumer Direct [Member] \| Nonaccrual [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	0		0		0
TDRs, amount	$ 0		$ 0		$ 0
Consumer Portfolio Segment [Member] \| Consumer Direct [Member] \| Accruing [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	23		23		23
TDRs, amount	$ 62		$ 62		$ 65
Consumer Portfolio Segment [Member] \| Home Equity [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	18		18		21
TDRs, amount	$ 430		$ 430		$ 437
Loans modified in TDR during the year, number \| Loan	1	0	3	3
Loans modified in TDR during the year, amount	$ 8	$ 0	$ 106	$ 48
Consumer Portfolio Segment [Member] \| Home Equity [Member] \| Nonaccrual [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	11		11		14
TDRs, amount	$ 223		$ 223		$ 221
Consumer Portfolio Segment [Member] \| Home Equity [Member] \| Accruing [Member]
Financing Receivable, Modifications [Line Items]
TDRs, number \| Loan	7		7		7
TDRs, amount	$ 207		$ 207		$ 216