INVESTMENT SECURITIES - Investment Securities Pledged as Collateral (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Investment Securities Pledged as Collateral
Investment securities pledged to collateralize certain deposits and borrowings	$ 2,480.0	$ 2,030.0
U.S. Treasury and Agency Securities [Member] \| Securities Sold under Agreements to Repurchase [Member]
Investment Securities Pledged as Collateral
Investment securities pledged to collateralize certain deposits and borrowings	$ 475.0	$ 473.4

Sum of the carrying amounts as of the date of the latest financial statement presented of securities which are owned but transferred to another party to serve as collateral to partially or fully secure a debt obligation, repurchase agreement or other current or potential obligation, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

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