Investment Securities - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments Schedule [Abstract]
Available-for-sale securities, amortized cost within one year	$ 0	$ 16,009
Available-for-sale securities, amortized cost after one year but within five years	20,051	19,052
Available-for-sale securities, amortized cost after five years but within ten years	179,812	276,635
Available-for-sale securities, amortized cost after ten years	647,928	495,439
Available-for-sale securities, amortized cost total	847,791	807,135
Available-for-sale securities, fair value within one year	0	16,017
Available-for-sale securities, fair value after one year but within five years	19,825	18,992
Available-for-sale securities, fair value after five years but within ten years	175,583	270,300
Available-for-sale securities, fair value after ten years	642,091	488,611
Available-for-sale securities, fair value total	837,499	793,920
Held-to-maturity securities, amortized cost within one year	322	0
Held-to-maturity securities, amortized cost after one year but within five years	5,895	6,168
Held-to-maturity securities, amortized cost after five years but within ten years	18,751	8,882
Held-to-maturity securities, amortized cost after ten years	136,218	149,296
Amortized Cost	161,186	164,346
Held-to-maturity securities, fair value within one year	320	0
Held-to-maturity securities, fair value after one year but within five years	5,894	6,162
Held-to-maturity securities, fair value after five years but within ten years	18,873	8,870
Held-to-maturity securities, fair value after ten years	137,766	148,200
Held-to-maturity securities, fair value total	$ 162,853	$ 163,232