Investment Securities - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities, amortized cost within one year	$ 8,995	$ 3,997
Available-for-sale securities, amortized cost after one year but within five years	26,075	30,009
Available-for-sale securities, amortized cost after five years but within ten years	281,237	218,023
Available-for-sale securities, amortized cost after ten years	443,024	472,043
Available-for-sale securities, amortized cost total	759,331	724,072
Available-for-sale securities, fair value within one year	9,010	3,995
Available-for-sale securities, fair value after one year but within five years	26,125	29,840
Available-for-sale securities, fair value after five years but within ten years	289,917	215,018
Available-for-sale securities, fair value after ten years	452,156	472,176
Available-for-sale securities, fair value total	777,208	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	5,097	3,465
Held-to-maturity securities, amortized cost after five years but within ten years	9,030	7,939
Held-to-maturity securities, amortized cost after ten years	150,753	152,972
Held-to-Maturity Securities, Amortized Cost	164,880	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	5,180	3,456
Held-to-maturity securities, after five years but within ten years	9,219	8,045
Held-to-maturity securities, after ten years	155,163	154,754
Held-to-maturity securities, fair value total	$ 169,562	$ 167,743