LOANS AND LEASES - Additional Information (Detail) $ in Millions			9 Months Ended		12 Months Ended
	Apr. 24, 2020 USD ($)	Jan. 01, 2020 USD ($)	Sep. 30, 2020 USD ($) State	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable			$ 64.5
Purchased with credit deterioration included with allowance for credit losses, amount		$ 50.3	50.3
Purchased with credit deterioration, discount			$ 110.0
Number of states, Company operating financial services | State			7
Transfer of indirect installment loans to loans held for sale			$ 508.0	$ 389.0
Allocation balance	$ 349,000.0
Additional authorized amount	$ 320,000.0
Payment deferral period with forgiveness determination	10 months
Loans granted short-term modification due to COVID-19			792.0
Restructured loans returned to performing status			6.9
Total loan on allowance		$ 196.0	373.0	$ 194.0	$ 196.0	$ 365.0	$ 188.0	$ 180.0
Pooled reserves for all other classes of loans			$ 4.6		4.1
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due period for loan to be in default			90 days
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due period for loan to be in default			12 months
SBA-Guaranteed Loan Servicing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fixed interest rate	1.00%
Term	2 years
Guaranteed percentage	100.00%
SBA-Guaranteed Loan Servicing [Member] | Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Processing fee paid by the SBA	1.00%
SBA-Guaranteed Loan Servicing [Member] | Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Processing fee paid by the SBA	5.00%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value of OREO through foreclosure			$ 2.9		3.3
Mortgage loans on real estate, foreclosure			10.7		9.2
Paycheck Protection Program Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans to customers			2,500.0
Commercial Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, collateral dependent			112.0
Commercial Loan [Member] | Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Threshold for loans whose terms have been modified in a trouble debt restructuring, non accrual			1.0
Total Commercial Loans and Leases [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loan on allowance			295.0		133.0	$ 279.0
Total Commercial Loans and Leases [Member] | Specific Reserves for Commercial Troubled Debt Restructurings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loan on allowance			1.6
Total Commercial Loans and Leases [Member] | Specific Reserves for Commercial Troubled Debt Restructurings [Member] | Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loan on allowance					0.5
Total Commercial Loans and Leases [Member] | Pooled Reserves For Commercial Troubled Debt Restructurings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loan on allowance			$ 3.4		$ 0.8