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Commitments and Letters of Credit - Notional Amount of Commitments and Letter of Credit Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss Contingencies [Line Items]
Commitments Net	$ 114	$ 358	$ 3,252
Commitments to extend credit
Loss Contingencies [Line Items]
Financial instrument of credit risk	1,840,180	1,733,820
Financial standby letters of credit
Loss Contingencies [Line Items]
Financial instrument of credit risk	17,946	18,108
Performance standby letters of credit
Loss Contingencies [Line Items]
Financial instrument of credit risk	20,472	26,630
Commercial letters of credit
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 1,149	$ 1,301