Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Commitments to extend credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 1,846,575	$ 1,840,180
Financial standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	18,486	17,946
Performance standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	21,460	20,472
Commercial letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 927	$ 1,149