LOANS AND LEASES - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended		12 Months Ended
	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jul. 01, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount		$ 21,839,403,000		$ 20,998,766,000
Reclassification from non-accretable difference		$ 184,545,000	$ 163,714,000
Sustained period of delinquency for impairment evaluation		90 years
Valuation for impairment of loans with pooled reserves		$ 1,000,000			$ 1,000,000
Minimum amount to allocate specific valuation allowance					$ 1,000,000
Interest income on impaired loans		4,300,000	$ 3,300,000
Restructured loans returned to performing status		3,000,000
Minimum reserves for commercial loan		1,000,000
Pooled reserves for all other classes of loans		$ 4,000,000		4,000,000
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]
Proceeds from sale acquired real estate loans	$ 56,500,000
Threshold period past due for default non-accrual status of trade accounts receivable		180 days
Carrying value of OREO through foreclosure		$ 5,400,000		3,600,000
Mortgage loans on real estate, foreclosure		9,000,000		15,200,000
Purchased Credit-Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount		$ 1,700,000		$ 1,900,000
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				0.03%
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%