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 - Schedule of Purchased Credit Impaired Loans for which Payments Would Not be Collected (Parenthetical) (Detail) - USD ($)	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable Impaired [Line Items]
Allowance for loan losses	$ 21,332,000	$ 21,405,000	$ 21,202,000	$ 20,555,000	$ 19,660,000	$ 19,087,000
Purchased Credit Impaired Loans [Member]
Financing Receivable Impaired [Line Items]
Allowance for loan losses	$ 0		$ 55,000