Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Commitments to extend credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 1,782,701	$ 1,840,180
Financial standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	17,636	17,946
Performance standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	21,328	20,472
Commercial letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 1,055	$ 1,149