Securities - Maturity of Debt Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Held for Investment, Contractual maturity of debt securities, Amortized Cost
Amortized cost, Within one year	$ 805	$ 300
Amortized cost, After one year but within five years	1,597	1,058
Amortized cost, After five years but within ten years	568	543
Amortized cost, After ten years	1	2
Amortized Cost	2,971	1,903
Available for sale, Contractual maturity of debt securities, Amortized Cost
Amortized Cost, Within one year	331,600	186,373
Amortized Cost, After one year but within five years	209,080	252,519
Amortized Cost, After five years but within ten years	3,850	5,873
Amortized Cost, After ten years	37,789	40,317
Amortized Cost	582,319	485,082
Held for Investment, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	811	300
Estimated Fair Value, After one year but within five years	1,631	1,055
Estimated Fair Value, After five years but within ten years	568	546
Estimated Fair Value, After ten years	1	2
Estimated Fair Value	3,011	1,903
Available for sale, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	332,496	186,539
Estimated Fair Value, After one year but within five years	219,983	257,430
Estimated Fair Value, After five years but within ten years	4,005	6,008
Estimated Fair Value, After ten years	37,486	39,746
Total debt securities	$ 593,970	$ 489,723