Securities - Amortized Cost and Fair Value of Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 5,099
Amortized Cost, Due after one year through five years	30,106
Amortized Cost, Due after five years through ten years	33,441
Amortized Cost, Due after ten years	71,769
Amortized Cost, Mortgage-backed securities	83,486
Amortized Cost, Equity securities	481
Amortized Cost	224,382	$ 192,820
Fair value, Due in one year or less	5,108
Fair value, Due after one year through five years	30,180
Fair value, Due after five years through ten years	35,104
Fair value, Due after ten years	74,286
Fair value, Mortgage-backed securities	83,923
Fair value, Equity securities	818
Fair value, Total securities available for sale	$ 229,419	$ 195,864