Investment Securities - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities, amortized cost within one year	$ 20,012	$ 16,009
Available-for-sale securities, amortized cost after one year but within five years	18,059	19,052
Available-for-sale securities, amortized cost after five years but within ten years	210,595	276,635
Available-for-sale securities, amortized cost after ten years	551,312	495,439
Available-for-sale securities, amortized cost total	799,978	807,135
Available-for-sale securities, fair value within one year	19,997	16,017
Available-for-sale securities, fair value after one year but within five years	17,981	18,992
Available-for-sale securities, fair value after five years but within ten years	205,242	270,300
Available-for-sale securities, fair value after ten years	545,291	488,611
Available-for-sale securities, fair value total	788,511	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	5,940	6,168
Held-to-maturity securities, amortized cost after five years but within ten years	10,482	8,882
Held-to-maturity securities, amortized cost after ten years	147,189	149,296
Held-to-Maturity Securities, Amortized Cost	163,611	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	5,975	6,162
Held-to-maturity securities, after five years but within ten years	10,511	8,870
Held-to-maturity securities, after ten years	146,755	148,200
Held-to-maturity securities, fair value total	$ 163,241	$ 163,232