Investment Securities - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities, amortized cost within one year	$ 10,994	$ 3,997
Available-for-sale securities, amortized cost after one year but within five years	28,086	30,009
Available-for-sale securities, amortized cost after five years but within ten years	239,335	218,023
Available-for-sale securities, amortized cost after ten years	467,333	472,043
Available-for-sale securities, amortized cost total	745,748	724,072
Available-for-sale securities, fair value within one year	11,009	3,995
Available-for-sale securities, fair value after one year but within five years	28,218	29,840
Available-for-sale securities, fair value after five years but within ten years	248,220	215,018
Available-for-sale securities, fair value after ten years	479,012	472,176
Available-for-sale securities, fair value total	766,459	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	3,179	3,465
Held-to-maturity securities, amortized cost after five years but within ten years	9,520	7,939
Held-to-maturity securities, amortized cost after ten years	153,699	152,972
Held-to-Maturity Securities, Amortized Cost	166,398	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	3,212	3,456
Held-to-maturity securities, after five years but within ten years	9,796	8,045
Held-to-maturity securities, after ten years	160,024	154,754
Held-to-maturity securities, fair value total	$ 173,032	$ 167,743