Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

LOANS AND LEASES - Reserve for Commercial TDRs Included in Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Modifications [Line Items]
Total allowance on acquired loans	$ 177,881	$ 176,574	$ 175,380	$ 170,016	$ 165,699	$ 158,059
Specific Reserves for Commercial TDRs [Member] \| Total Commercial Loans and Leases [Member]
Financing Receivable, Modifications [Line Items]
Total allowance on acquired loans	0		95
Pooled Reserves for Individual Commercial Loans [Member] \| Total Commercial Loans and Leases [Member]
Financing Receivable, Modifications [Line Items]
Total allowance on acquired loans	$ 551		$ 469