Securities - Amortized Cost and Estimated Fair Value of Investment Securities by Contractual Maturity (Detail) $ in Thousands	Mar. 31, 2018 USD ($)
Investments, Debt and Equity Securities [Abstract]
Less than one year, Amortized Cost	$ 56,691
Due after one year through five years, Amortized Cost	152,009
Due after five years through ten years, Amortized Cost	91,202
Due after ten years, Amortized Cost	35,751
Total Amortized Cost	335,653
Less than one year, Estimated Fair Value	56,586
Due after one year through five years, Estimated Fair Value	149,604
Due after five years through ten years, Estimated Fair Value	87,763
Due after ten years, Estimated Fair Value	34,424
Total Estimated Fair Value	$ 328,377