Securities - Maturity of Debt Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Held for Investment, Contractual maturity of debt securities, Amortized Cost
Amortized cost, Within one year	$ 807	$ 300
Amortized cost, After one year but within five years	2,091	1,058
Amortized cost, After five years but within ten years	65	543
Amortized cost, After ten years	1	2
Amortized Cost	2,964	1,903
Available for sale, Contractual maturity of debt securities, Amortized Cost
Amortized Cost, Within one year	339,752	186,373
Amortized Cost, After one year but within five years	162,401	252,519
Amortized Cost, After five years but within ten years	3,753	5,873
Amortized Cost, After ten years	36,383	40,317
Amortized Cost	542,289	485,082
Held for Investment, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	809	300
Estimated Fair Value, After one year but within five years	2,110	1,055
Estimated Fair Value, After five years but within ten years	64	546
Estimated Fair Value, After ten years	1	2
Estimated Fair Value	2,984	1,903
Available for sale, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	341,102	186,539
Estimated Fair Value, After one year but within five years	171,135	257,430
Estimated Fair Value, After five years but within ten years	3,910	6,008
Estimated Fair Value, After ten years	36,085	39,746
Total debt securities	$ 552,232	$ 489,723