Securities - Maturity of Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Held for Investment, Contractual maturity of debt securities, Amortized Cost
Amortized cost, Within one year	$ 4,978	$ 1,451
Amortized cost, After one year but within five years	3,003	6,603
Amortized cost, After five years but within ten years	316	380
Amortized cost, After ten years	36	159
Amortized Cost	8,333	8,593
Available for sale, Contractual maturity of debt securities, Amortized Cost
Amortized Cost, Within one year	132,557	41,772
Amortized Cost, After one year but within five years	266,217	350,975
Amortized Cost, After five years but within ten years	12,493	21,990
Amortized Cost, After ten years	77,590	87,252
Total debt securities	488,857	501,989
Amortized Cost, Equity securities	6,433	7,364
Amortized Cost	495,290	509,353
Held for Investment, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	4,985	1,456
Estimated Fair Value, After one year but within five years	3,031	6,642
Estimated Fair Value, After five years but within ten years	343	396
Estimated Fair Value, After ten years	39	177
Estimated Fair Value	8,398	8,671
Available for sale, Contractual maturity of debt securities, Estimated Fair Value
Estimated Fair Value, Within one year	132,794	41,870
Estimated Fair Value, After one year but within five years	269,077	352,044
Estimated Fair Value, After five years but within ten years	13,068	22,717
Estimated Fair Value, After ten years	78,155	88,132
Total debt securities	493,094	504,763
Estimated Fair Value, Equity securities	6,431	11,427
Estimated Fair Value, Total	$ 499,525	$ 516,190