Investment Securities Held-to-Maturity - Company's Securities Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Within one year, Amortized Cost	$ 10,047
After one but within five years, Amortized Cost	1,400,716
After five but within ten years, Amortized Cost	159,917
More than ten years, Amortized Cost	3,149
Amortized Cost	1,573,829	$ 1,438,903
Within one year, Fair Value	10,137
After one but within five years, Fair Value	1,427,594
After five but within ten years, Fair Value	165,817
More than ten years, Fair Value	3,210
Estimated Fair Value	$ 1,606,758	$ 1,438,960