LOANS AND LEASES - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount	$ 22,620.0		$ 22,153.0
Reclassification from non-accretable difference	$ 30.0	$ 64.2
Sustained period of delinquency for impairment evaluation	90 days
Valuation for impairment of loans with pooled reserves	$ 1.0
Minimum amount to allocate specific valuation allowance	1.0
Interest income on impaired loans	1.4	1.6
Restructured loans returned to performing status	1.3
Minimum reserves for commercial loan	1.0
Total allowance on loans acquired in a business combination	186.0	179.0	180.0	$ 175.0
Pooled reserves for all other classes of loans	$ 3.9		4.0
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
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Threshold period past due for default non-accrual status of trade accounts receivable	90 days
Installment Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Threshold period past due for default non-accrual status of trade accounts receivable	120 days
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Threshold period past due for default non-accrual status of trade accounts receivable	180 days
Carrying value of OREO through foreclosure	$ 4.4		6.3
Mortgage loans on real estate, foreclosure	$ 9.3		$ 8.9
Loan Portfolio Diversification Risk [Member] | Residential Construction Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of portfolio	1.50%		1.20%
Purchased Credit-Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount	$ 1.7		$ 1.7
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			0.04%
Purchased Credit-Impaired Loans [Member] | Credit Concentration Risk [Member] | Acquired Loans Receivable [Member] | Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of portfolio	0.04%
Residential Construction Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount	$ 329.9		$ 273.4
Total Commercial Loans and Leases [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount	14,147.0		$ 13,761.0
Total Commercial Loans and Leases [Member] | Specific Reserves for Commercial TDRs [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total allowance on loans acquired in a business combination	0.5	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 allowance on loans acquired in a business combination	$ 0.9	$ 0.9