Investment Securities - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities, amortized cost within one year	$ 2,993	$ 16,009
Available-for-sale securities, amortized cost after one year but within five years	10,043	19,052
Available-for-sale securities, amortized cost after five years but within ten years	195,075	276,635
Available-for-sale securities, amortized cost after ten years	616,740	495,439
Available-for-sale securities, amortized cost total	824,851	807,135
Available-for-sale securities, fair value within one year	2,993	16,017
Available-for-sale securities, fair value after one year but within five years	10,062	18,992
Available-for-sale securities, fair value after five years but within ten years	191,104	270,300
Available-for-sale securities, fair value after ten years	613,716	488,611
Available-for-sale securities, fair value total	817,875	793,920
Held-to-maturity securities, amortized cost within one year	0	0
Held-to-maturity securities, amortized cost after one year but within five years	5,906	6,168
Held-to-maturity securities, amortized cost after five years but within ten years	13,142	8,882
Held-to-maturity securities, amortized cost after ten years	143,550	149,296
Held-to-Maturity Securities, Amortized Cost	162,598	164,346
Held-to-maturity securities, fair value within one year	0	0
Held-to-maturity securities, fair value after one year but within five years	5,949	6,162
Held-to-maturity securities, after five years but within ten years	13,276	8,870
Held-to-maturity securities, after ten years	144,678	148,200
Held-to-maturity securities, fair value total	$ 163,903	$ 163,232