Financial Instruments with Off-Balance-Sheet Risk and Concentrations of Credit Risk - Summary of Approximate Contract Amounts of Collateral or Other Security to Support Financial Instruments with Credit Risk (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Commitments to extend credit
Schedule Of Financial Instruments With Off Balance Sheet Credit Risk [Line Items]
Financial instruments with off-balance sheet risks, contract amount	$ 966,441	$ 921,979
Standby letters of credit and financial guarantees written
Schedule Of Financial Instruments With Off Balance Sheet Credit Risk [Line Items]
Financial instruments with off-balance sheet risks, contract amount	$ 14,832	$ 15,170