Securities - Maturity of Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Held for Investment, Contractual maturity of debt securities, Amortized Cost
Amortized cost, Within one year	$ 4,987	$ 5,168
Amortized cost, After one year but within five years	2,543	2,800
Amortized cost, After five years but within ten years	760	795
Amortized cost, After ten years	22	26
Amortized Cost	8,312	8,789
Available for sale, Contractual maturity of debt securities, Amortized Cost
Amortized Cost, Within one year	182,254	272,820
Amortized Cost, After one year but within five years	140,232	178,617
Amortized Cost, After five years but within ten years	7,986	8,483
Amortized Cost, After ten years	69,695	75,522
Total debt securities	400,167	535,442
Amortized Cost, Equity securities	6,514	6,228
Amortized Cost	406,681	541,670
Held for Investment, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	4,997	5,174
Estimated Fair Value, After one year but within five years	2,569	2,829
Estimated Fair Value, After five years but within ten years	784	319
Estimated Fair Value, After ten years	22	528
Estimated Fair Value	8,372	8,850
Available for sale, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	182,608	272,779
Estimated Fair Value, After one year but within five years	143,197	180,145
Estimated Fair Value, After five years but within ten years	8,610	9,075
Estimated Fair Value, After ten years	70,104	75,853
Total debt securities	404,519	537,852
Estimated Fair Value, Equity securities	6,407	6,308
Estimated Fair Value, Total	$ 410,926	$ 544,160