Loans - Schedule of Purchased Credit Impaired Loans for which Payments Would Not be Collected (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable Impaired [Line Items]
Allowance for loan losses	$ 21,202	$ 19,087	$ 17,712	$ 17,687
Purchased Credit Impaired Loans [Member]
Financing Receivable Impaired [Line Items]
Allowance for loan losses	$ 55