Securities - Maturity of Debt Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Held for Investment, Contractual maturity of debt securities, Amortized Cost
Amortized cost, Within one year	$ 862	$ 807
Amortized cost, After one year but within five years	2,089	2,091
Amortized cost, After five years but within ten years	4	65
Amortized cost, After ten years	1	1
Amortized Cost	2,956	2,964
Available for sale, Contractual maturity of debt securities, Amortized Cost
Amortized Cost, Within one year	110,317	339,752
Amortized Cost, After one year but within five years	359,388	162,401
Amortized Cost, After five years but within ten years	5,914	3,753
Amortized Cost, After ten years	33,367	36,383
Amortized Cost	508,986	542,289
Held for Investment, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	862	809
Estimated Fair Value, After one year but within five years	2,085	2,110
Estimated Fair Value, After five years but within ten years	5	64
Estimated Fair Value, After ten years	1	1
Estimated Fair Value	2,953	2,984
Available for sale, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	111,484	341,102
Estimated Fair Value, After one year but within five years	366,433	171,135
Estimated Fair Value, After five years but within ten years	6,049	3,910
Estimated Fair Value, After ten years	33,621	36,085
Total debt securities	$ 517,587	$ 552,232