Securities - Maturity of Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Held for Investment, Contractual maturity of debt securities, Amortized Cost
Amortized cost, Within one year	$ 1,036	$ 1,036
Amortized cost, After one year but within five years	422	623
Amortized cost, After five years but within ten years	613	625
Amortized cost, After ten years	8	8
Amortized Cost	2,079	2,292
Available for sale, Contractual maturity of debt securities, Amortized Cost
Amortized Cost, Within one year	162,621	113,225
Amortized Cost, After one year but within five years	268,010	289,038
Amortized Cost, After five years but within ten years	5,616	6,222
Amortized Cost, After ten years	52,671	55,813
Total debt securities	488,918	464,298
Amortized Cost, Equity securities	7,088	6,527
Amortized Cost	496,006	470,825
Held for Investment, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	1,036	1,034
Estimated Fair Value, After one year but within five years	422	627
Estimated Fair Value, After five years but within ten years	620	633
Estimated Fair Value, After ten years	8	9
Estimated Fair Value	2,086	2,303
Available for sale, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	162,261	112,974
Estimated Fair Value, After one year but within five years	264,525	287,058
Estimated Fair Value, After five years but within ten years	5,819	6,500
Estimated Fair Value, After ten years	52,152	55,467
Total debt securities	484,757	461,999
Estimated Fair Value, Equity securities	6,265	5,704
Estimated Fair Value, Total	$ 491,022	$ 467,703