Investment Securities Held-to-Maturity - Company's Securities Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Within one year, Amortized Cost	$ 15,191
After one but within five years, Amortized Cost	1,360,309
After five but within ten years, Amortized Cost	186,544
More than ten years, Amortized Cost	3,147
Amortized Cost	1,565,191	$ 1,438,903
Within one year, Fair Value	15,314
After one but within five years, Fair Value	1,379,055
After five but within ten years, Fair Value	191,613
More than ten years, Fair Value	3,201
Estimated Fair Value	$ 1,589,183	$ 1,438,960