Securities - Maturity of Debt Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Held for Investment, Contractual maturity of debt securities, Amortized Cost
Amortized cost, Within one year	$ 300	$ 495
Amortized cost, After one year but within five years	1,146	369
Amortized cost, After five years but within ten years	549	562
Amortized cost, After ten years	2	2
Amortized Cost	1,997	1,428
Available for sale, Contractual maturity of debt securities, Amortized Cost
Amortized Cost, Within one year	103,395	411,256
Amortized Cost, After one year but within five years	270,900	313,416
Amortized Cost, After five years but within ten years	6,355	7,524
Amortized Cost, After ten years	37,603	41,378
Amortized Cost	418,253	773,574
Held for Investment, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	301	495
Estimated Fair Value, After one year but within five years	1,147	370
Estimated Fair Value, After five years but within ten years	553	565
Estimated Fair Value, After ten years	2	3
Estimated Fair Value	2,003	1,433
Available for sale, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	103,084	410,327
Estimated Fair Value, After one year but within five years	276,333	311,924
Estimated Fair Value, After five years but within ten years	6,606	7,685
Estimated Fair Value, After ten years	37,134	40,768
Total debt securities	$ 423,157	$ 770,704