Investment Securities Held-to-Maturity - Company's Securities Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Within one year, Amortized Cost	$ 9,242
After one but within five years, Amortized Cost	1,205,411
After five but within ten years, Amortized Cost	193,283
More than ten years, Amortized Cost	4,766
Amortized Cost	1,412,702	$ 1,438,903
Within one year, Fair Value	9,343
After one but within five years, Fair Value	1,223,805
After five but within ten years, Fair Value	199,272
More than ten years, Fair Value	4,785
Estimated Fair Value	$ 1,437,205	$ 1,438,960