Securities - Maturity of Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Held for Investment, Contractual maturity of debt securities, Amortized Cost
Amortized cost, Within one year	$ 495	$ 1,036
Amortized cost, After one year but within five years	370	623
Amortized cost, After five years but within ten years	570	625
Amortized cost, After ten years	5	8
Amortized Cost	1,440	2,292
Available for sale, Contractual maturity of debt securities, Amortized Cost
Amortized Cost, Within one year	61,869	113,225
Amortized Cost, After one year but within five years	368,309	289,038
Amortized Cost, After five years but within ten years	5,047	6,222
Amortized Cost, After ten years	46,684	55,813
Total debt securities	481,909	464,298
Amortized Cost, Equity securities		6,527
Amortized Cost	481,909	470,825
Held for Investment, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	496	1,034
Estimated Fair Value, After one year but within five years	371	627
Estimated Fair Value, After five years but within ten years	575	633
Estimated Fair Value, After ten years	5	9
Estimated Fair Value	1,447	2,303
Available for sale, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	61,656	112,974
Estimated Fair Value, After one year but within five years	362,723	287,058
Estimated Fair Value, After five years but within ten years	5,225	6,500
Estimated Fair Value, After ten years	46,036	55,467
Total debt securities	475,640	461,999
Estimated Fair Value, Equity securities		5,704
Estimated Fair Value, Total	$ 475,640	$ 467,703