Securities - Maturity of Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Held for Investment, Contractual maturity of debt securities, Amortized Cost
Amortized cost, Within one year	$ 1,672	$ 1,561
Amortized cost, After one year but within five years	1,593	1,937
Amortized cost, After five years but within ten years	667	707
Amortized cost, After ten years	10	160
Amortized Cost	3,942	4,365
Available for sale, Contractual maturity of debt securities, Amortized Cost
Amortized Cost, Within one year	66,022	66,542
Amortized Cost, After one year but within five years	308,283	320,150
Amortized Cost, After five years but within ten years	5,849	5,830
Amortized Cost, After ten years	60,129	66,199
Total debt securities	440,283	458,721
Amortized Cost, Equity securities	6,450	6,594
Amortized Cost	446,733	465,315
Held for Investment, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	1,683	1,568
Estimated Fair Value, After one year but within five years	1,603	1,951
Estimated Fair Value, After five years but within ten years	674	723
Estimated Fair Value, After ten years	15	161
Estimated Fair Value	3,975	4,403
Available for sale, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	66,105	66,662
Estimated Fair Value, After one year but within five years	308,913	319,839
Estimated Fair Value, After five years but within ten years	6,184	6,152
Estimated Fair Value, After ten years	60,303	66,246
Total debt securities	441,505	458,899
Estimated Fair Value, Equity securities	5,955	6,569
Estimated Fair Value, Total	$ 447,460	$ 465,468