Investment Securities - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities, amortized cost within one year	$ 3,997	$ 10,000
Available-for-sale securities, amortized cost after one year but within five years	30,009	20,020
Available-for-sale securities, amortized cost after five years but within ten years	218,023	134,453
Available-for-sale securities, amortized cost after ten years	472,043	574,931
Available-for-sale securities, amortized cost total	724,072	739,404
Available-for-sale securities, fair value within one year	3,995	10,014
Available-for-sale securities, fair value after one year but within five years	29,840	19,946
Available-for-sale securities, fair value after five years but within ten years	215,018	133,395
Available-for-sale securities, fair value after ten years	472,176	576,769
Available-for-sale securities, fair value total	721,029	740,124
Held-to-maturity securities, amortized cost within one year	1,486	3,608
Held-to-maturity securities, amortized cost after one year but within five years	3,465	6,217
Held-to-maturity securities, amortized cost after five years but within ten years	7,939	9,733
Held-to-maturity securities, amortized cost after ten years	152,972	106,610
Held-to-Maturity Securities, Amortized Cost	165,862	126,168
Held-to-maturity securities, fair value within one year	1,488	3,608
Held-to-maturity securities, fair value after one year but within five years	3,456	6,217
Held-to-maturity securities, after five years but within ten years	8,045	9,736
Held-to-maturity securities, after ten years	154,754	106,610
Held-to-maturity securities, fair value total	$ 167,743	$ 126,171