Investment Securities - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities, amortized cost within one year	$ 16,009	$ 3,997
Available-for-sale securities, amortized cost after one year but within five years	19,052	30,009
Available-for-sale securities, amortized cost after five years but within ten years	276,635	218,023
Available-for-sale securities, amortized cost after ten years	495,439	472,043
Available-for-sale securities, amortized cost total	807,135	724,072
Available-for-sale securities, fair value within one year	16,017	3,995
Available-for-sale securities, fair value after one year but within five years	18,992	29,840
Available-for-sale securities, fair value after five years but within ten years	270,300	215,018
Available-for-sale securities, fair value after ten years	488,611	472,176
Available-for-sale securities, fair value total	793,920	721,029
Held-to-maturity securities, amortized cost within one year		1,486
Held-to-maturity securities, amortized cost after one year but within five years	6,168	3,465
Held-to-maturity securities, amortized cost after five years but within ten years	8,882	7,939
Held-to-maturity securities, amortized cost after ten years	149,296	152,972
Held-to-Maturity Securities, Amortized Cost	164,346	165,862
Held-to-maturity securities, fair value within one year		1,488
Held-to-maturity securities, fair value after one year but within five years	6,162	3,456
Held-to-maturity securities, fair value after five years but within ten years	8,870	8,045
Held-to-maturity securities, fair value after ten years	148,200	154,754
Held-to-maturity securities, fair value total	$ 163,232	$ 167,743