COMMITMENTS AND CONTINGENCIES - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Commitments and Contingencies Disclosure [Line Items]
Reserves for unfunded commitments	$ 11.6		$ 11.6		$ 12.5
Loans and Leases Receivable, Commitments, Fixed Rates			158.8		123.6
Loans and Leases Receivable, Commitments, Variable Rates			$ 3,600.0		$ 3,300.0
Loan Commitments, Fixed Interest Rate Range, Minimum			0.00%		0.00%
Loan Commitments, Fixed Interest Rate Range, Maximum			21.00%		21.00%
Loan Commitments, Fixed Rate, Maturities, Minimum			1 year		1 year
Loan Commitments, Fixed Rate, Maturities, Maximum			30 years		30 years 9 months 18 days
Letters of credit issued to guarantee performance of a client to a third party	36.2		$ 36.2		$ 36.1
Litigation Settlement, Expense	0.0	$ 0.0	3.8	$ 0.0
Estimated Litigation Liability	0.0		0.0		0.0
Commitments to Extend Credit
Commitments and Contingencies Disclosure [Line Items]
Commitments outstanding to extend credit	$ 3,700.0		$ 3,700.0		$ 3,400.0

Commitments and Contingencies Disclosure [Line Items]

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- Definition

Maximum interest rate range of loan commitments with fixed interest rates.

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Loan commitments with a fixed interest rate minimum interest rate range.

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The maximum maturity of fixed rate loan commitments.

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No definition available.

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The minimum maturity of fixed rate loan commitments.

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Reserves for unfunded commitments

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- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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No definition available.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Aggregate carrying amount of the estimated litigation liability for known or estimated probable loss from litigation, which may include attorneys' fees and other litigation costs.

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Amount of litigation expense, including but not limited to legal, forensic, accounting, and investigative fees.

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Reflects the amount of cash required to fully fund agreements to provide advances to borrowers and lessees at stated interest rates.

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Reflects the amount of cash required to fully fund agreements to provide advances to borrowers and lessees at interest rates that increase or decrease depending on certain facts or circumstances.

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