Securities - Amortized Cost and Fair Value of Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 13,504
Amortized Cost, Due after one year through five years	22,556
Amortized Cost, Due after five years through ten years	29,983
Amortized Cost, Due after ten years	79,632
Amortized Cost, Mortgage-backed securities	85,556
Amortized Cost	231,231
Fair value, Due in one year or less	13,462
Fair value, Due after one year through five years	22,415
Fair value, Due after five years through ten years	30,994
Fair value, Due after ten years	80,176
Fair value, Mortgage-backed securities	83,966
Fair value, Total securities available for sale	$ 231,013	$ 230,230