Commitments and Contingencies - Off-balance Sheet Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Off-balance Sheet Risks, Amount, Liability	$ 390	$ 385
Commitments to Extend Credit [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Off-balance Sheet Risks, Amount, Liability	1,127,545
Performance Letters Of Credit [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Off-balance Sheet Risks, Amount, Liability	27,634
Standby Letters of Credit [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Off-balance Sheet Risks, Amount, Liability	24,477
Other Letters Of Credit [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Off-balance Sheet Risks, Amount, Liability	$ 849