Securities - Maturity of Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Held for Investment, Contractual maturity of debt securities, Amortized Cost
Amortized cost, Within one year	$ 1,561	$ 5,168
Amortized cost, After one year but within five years	1,937	2,800
Amortized cost, After five years but within ten years	707	795
Amortized cost, After ten years	160	26
Amortized Cost	4,365	8,789
Available for sale, Contractual maturity of debt securities, Amortized Cost
Amortized Cost, Within one year	66,542	272,820
Amortized Cost, After one year but within five years	320,150	178,617
Amortized Cost, After five years but within ten years	5,830	8,483
Amortized Cost, After ten years	66,199	75,522
Total debt securities	458,721	535,442
Amortized Cost, Other securities	6,594	6,228
Amortized Cost	465,315	541,670
Held for Investment, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	1,568	5,174
Estimated Fair Value, After one year but within five years	1,951	2,829
Estimated Fair Value, After five years but within ten years	723	319
Estimated Fair Value, After ten years	161	528
Estimated Fair Value	4,403	8,850
Available for sale, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	66,662	272,779
Estimated Fair Value, After one year but within five years	319,839	180,145
Estimated Fair Value, After five years but within ten years	6,152	9,075
Estimated Fair Value, After ten years	66,246	75,853
Total debt securities	458,899	537,852
Estimated Fair Value, Other securities	6,569	6,308
Estimated Fair Value, Total	$ 465,468	$ 544,160