The cash inflow from the exercise of the rights to redeem debentures and trust preferred securities, net of the cash outflow for repurchasing trust preferred securities, by a business trust or other special purpose entity, mainly established by a bank holding entity, to third party investors. The trust's assets are deeply subordinated debentures of the bank holding entity. Most trust preferred securities are subject to a mandatory redemption upon the repayment of the debentures.

No definition available.

Name:	cbu_ProceedsFromRedemptionOfDebenturesAndPreferredSecurities
Namespace Prefix:	cbu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

BORROWINGS, Preferred Securities (Details) - USD ($) $ in Millions	Mar. 15, 2021	Sep. 15, 2020
Community Capital Trust IV [Member]
Mandatorily Redeemable Preferred Securities [Abstract]
Redemption of debentures and associated preferred securities	$ 77.3
Kinderhook Capital Trust [Member]
Mandatorily Redeemable Preferred Securities [Abstract]
Redemption of debentures and associated preferred securities		$ 2.1