Securities - Amortized Cost and Estimated Fair Value of Investment Securities by Contractual Maturity (Detail) $ in Thousands	Sep. 30, 2018 USD ($)
Investments, Debt and Equity Securities [Abstract]
Less than one year, Amortized Cost	$ 56,753
Due after one year through five years, Amortized Cost	156,879
Due after five years through ten years, Amortized Cost	90,804
Due after ten years, Amortized Cost	10,795
Total Amortized Cost	315,231
Less than one year, Estimated Fair Value	56,522
Due after one year through five years, Estimated Fair Value	153,904
Due after five years through ten years, Estimated Fair Value	86,373
Due after ten years, Estimated Fair Value	11,664
Total Estimated Fair Value	$ 308,463