FAIR VALUE - Financial assets and liabilities accounted for at fair value on a recurring basis (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023
Available-for-sale investment securities
Fair Value	$ 2,785,714	$ 2,919,992
Unpaid principal value of mortgage loans held for sale	1,500	400
Fair value, assets, levels transfers	0
Transfers between Level 2 and Level 1	0
Transfers in of level 3	0
Transfers out of level 3	0
U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	2,083,786	2,080,783
Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	386,495	474,363
Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	301,224	348,526
Corporate debt securities
Available-for-sale investment securities
Fair Value	7,697	7,394
Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	6,512	8,926
Recurring
Available-for-sale investment securities
Fair Value	2,785,714	2,919,992
Equity securities	2,354	372
Mortgage loans held for sale	1,470	414
Total	2,789,745	2,920,786
Recurring \| Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	215	2
Recurring \| Interest rate swap agreements
Available-for-sale investment securities
Derivative asset	2,664
Derivative liability	(2,664)
Recurring \| Forward sales commitments
Available-for-sale investment securities
Derivative asset	(8)	6
Recurring \| U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	2,083,786	2,080,783
Recurring \| Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	386,495	474,363
Recurring \| Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	301,224	348,526
Recurring \| Corporate debt securities
Available-for-sale investment securities
Fair Value	7,697	7,394
Recurring \| Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	6,512	8,926
Recurring \| Level 1
Available-for-sale investment securities
Fair Value	2,025,164	2,019,089
Equity securities	2,354	372
Mortgage loans held for sale	0	0
Total	2,027,518	2,019,461
Recurring \| Level 1 \| Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	0	0
Recurring \| Level 1 \| Interest rate swap agreements
Available-for-sale investment securities
Derivative asset	0
Derivative liability	0
Recurring \| Level 1 \| Forward sales commitments
Available-for-sale investment securities
Derivative asset	0	0
Recurring \| Level 1 \| U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	2,025,164	2,019,089
Recurring \| Level 1 \| Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 1 \| Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 1 \| Corporate debt securities
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 1 \| Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 2
Available-for-sale investment securities
Fair Value	760,550	900,903
Equity securities	0	0
Mortgage loans held for sale	1,470	414
Total	762,012	901,323
Recurring \| Level 2 \| Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	0	0
Recurring \| Level 2 \| Interest rate swap agreements
Available-for-sale investment securities
Derivative asset	2,664
Derivative liability	(2,664)
Recurring \| Level 2 \| Forward sales commitments
Available-for-sale investment securities
Derivative asset	(8)	6
Recurring \| Level 2 \| U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	58,622	61,694
Recurring \| Level 2 \| Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	386,495	474,363
Recurring \| Level 2 \| Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	301,224	348,526
Recurring \| Level 2 \| Corporate debt securities
Available-for-sale investment securities
Fair Value	7,697	7,394
Recurring \| Level 2 \| Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	6,512	8,926
Recurring \| Level 3
Available-for-sale investment securities
Fair Value	0	0
Equity securities	0	0
Mortgage loans held for sale	0	0
Total	215	2
Recurring \| Level 3 \| Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	215	2
Recurring \| Level 3 \| Interest rate swap agreements
Available-for-sale investment securities
Derivative asset	0
Derivative liability	0
Recurring \| Level 3 \| Forward sales commitments
Available-for-sale investment securities
Derivative asset	0	0
Recurring \| Level 3 \| U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 3 \| Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 3 \| Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 3 \| Corporate debt securities
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 3 \| Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	$ 0	$ 0

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of transfers of liabilities measured on a recurring basis out of Levels 2 of the fair value hierarchy into Level 1.

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

Amount of unpaid principal value of mortgage loans held-for sale.

+ References

No definition available.

+ Details

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Namespace Prefix:	cbu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

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Period Type:	duration

- Definition

Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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