Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 607
Due after one year through five years, Amortized Cost	14,853
Due after five years through ten years, Amortized Cost	91,635
Due after ten years, Amortized Cost	839,656
Totals, Amortized Cost	946,751
Due in one year, Estimated Fair Value	611
Due after one year through five years, Estimated Fair Value	15,204
Due after five years through ten years, Estimated Fair Value	91,739
Due after ten years, Estimated Fair Value	842,584
Totals, Estimated Fair Value	950,138
Due in one year, Amortized Cost, Held to Maturity	1,271
Due after one year through five years, Amortized Cost, Held to Maturity	0
Due after five years through ten years, Amortized Cost, Held to Maturity	21,366
Due after ten years, Amortized Cost, Held to Maturity	352,969
Totals, Amortized Cost, Held to Maturity	375,606	$ 444,936
Due in one year, Estimated Fair Value, Held to Maturity	1,280
Due after one year through five years, Estimated Fair Value, Held to Maturity	0
Due after five years through ten years, Estimated Fair Value, Held to Maturity	21,639
Due after ten years, Estimated Fair Value, Held to Maturity	358,606
Estimated Fair Value, Held to Maturity	$ 381,525	$ 437,370