LOANS AND LEASES - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2020	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable		$ 60.9
Purchased with credit deterioration included with allowance for credit losses, amount	$ 50.3	50.3
Purchased with credit deterioration, discount		$ 110.0
Sustained period of delinquency for impairment evaluation		90 days
Valuation for impairment of loans with pooled reserves		$ 1.0
Interest income on impaired loans			$ 1.4
Restructured loans returned to performing status		3.7
Total loan on allowance	$ 196.0	343.0	186.0	$ 196.0	$ 180.0
Pooled reserves for all other classes of loans		4.9		4.1
Amortized cost loans and leases, carrying amount		$ 23,871.0		23,289.0
Reclassification from non-accretable difference			30.0
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due period for loan to be in default		12 months
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due period for loan to be in default		90 days
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value of OREO through foreclosure		$ 3.2		3.3
Mortgage loans on real estate, foreclosure		10.7	$ 9.2
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortized cost loans and leases, carrying amount				1.5
Purchased Credit Impaired Loans [Member] | Credit Concentration Risk [Member] | Acquired Loans Receivable [Member] | Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of portfolio			0.05%
Commercial Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, collateral dependent		40.0
Minimum amount to allocate specific valuation allowance		1.0
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		254.0		133.0
Amortized cost loans and leases, carrying amount		15,260.0		14,721.0
Total Commercial Loans and Leases [Member] | Specific and Pooled Reserves for Commercial Troubled Debt Restructurings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loan on allowance		1.0
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		1.1		1.1
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		$ 3.1		$ 0.8