Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Commitments to extend credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 1,776,677	$ 1,733,820
Financial standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	17,979	18,108
Performance standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	25,704	26,630
Commercial letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 837	$ 1,301