Securities - Amortized Cost and Estimated Fair Value of Investment Securities by Contractual Maturity (Detail) $ in Thousands	Sep. 30, 2017 USD ($)
Investments, Debt and Equity Securities [Abstract]
Less than one year, Amortized Cost	$ 35,395
Due after one year through five years, Amortized Cost	145,590
Due after five years through ten years, Amortized Cost	89,509
Due after ten years, Amortized Cost	30,000
Total Amortized Cost	300,494
Less than one year, Estimated Fair Value	35,384
Due after one year through five years, Estimated Fair Value	145,528
Due after five years through ten years, Estimated Fair Value	88,277
Due after ten years, Estimated Fair Value	27,751
Total Estimated Fair Value	$ 296,940