Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Weighted Average Yield
Within one year or less	1.74%
One through five years	1.67%
After five through ten years	2.32%
Over ten years	2.26%
Weighted average yield	2.22%
Amortized Cost
Within one year or less	$ 27,692
One through five years	283,137
After five through ten years	789,829
Over ten years	2,756,555
Amortized cost	3,857,213
Estimated Fair Value
Within one year or less	27,625
One through five years	283,609
After five through ten years	786,876
Over ten years	2,725,843
Estimated fair value	$ 3,823,953	$ 3,446,097
Weighted Average Yield
Within one year or less	2.87%
One through five years	2.90%
After five through ten years	3.08%
Over ten years	2.83%
Weighted average yield	2.91%
Amortized Cost
Within one year or less	$ 1,896
One through five years	8,274
After five through ten years	22,252
Over ten years	53,596
Amortized cost	86,018	89,216
Estimated Fair Value
Within one year or less	1,907
One through five years	8,454
After five through ten years	22,852
Over ten years	53,600
Estimated fair value	$ 86,813	$ 89,932