Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Commitments to extend credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 1,881,445	$ 1,883,914
Financial standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	18,121	18,298
Performance standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	28,520	22,027
Commercial letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 887	$ 887