Securities - Amortized Cost and Estimated Fair Value of Investment Securities by Contractual Maturity (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Investments, Debt and Equity Securities [Abstract]
Less than one year, Amortized Cost	$ 50,564
Due after one year through five years, Amortized Cost	159,365
Due after five years through ten years, Amortized Cost	85,574
Due after ten years, Amortized Cost	10,817
Total Amortized Cost	306,320
Less than one year, Estimated Fair Value	50,346
Due after one year through five years, Estimated Fair Value	157,606
Due after five years through ten years, Estimated Fair Value	80,049
Due after ten years, Estimated Fair Value	10,901
Total Estimated Fair Value	$ 298,902