Securities - Maturity of Debt Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Held for Investment, Contractual maturity of debt securities, Amortized Cost
Amortized cost, Within one year	$ 295	$ 495
Amortized cost, After one year but within five years	368	369
Amortized cost, After five years but within ten years	552	562
Amortized cost, After ten years	2	2
Amortized Cost	1,217	1,428
Available for sale, Contractual maturity of debt securities, Amortized Cost
Amortized Cost, Within one year	403,084	411,256
Amortized Cost, After one year but within five years	273,033	313,416
Amortized Cost, After five years but within ten years	7,265	7,524
Amortized Cost, After ten years	39,735	41,378
Amortized Cost	723,117	773,574
Held for Investment, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	295	495
Estimated Fair Value, After one year but within five years	370	370
Estimated Fair Value, After five years but within ten years	556	565
Estimated Fair Value, After ten years	2	3
Estimated Fair Value	1,223	1,433
Available for sale, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	402,210	410,327
Estimated Fair Value, After one year but within five years	274,838	311,924
Estimated Fair Value, After five years but within ten years	7,487	7,685
Estimated Fair Value, After ten years	39,120	40,768
Total debt securities	$ 723,655	$ 770,704