Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Weighted Average Yield
Within one year or less	1.61%
One through five years	1.78%
After five through ten years	2.34%
Over ten years	2.27%
Weighted average yield	2.25%
Amortized Cost
Within one year or less	$ 22,066
One through five years	250,400
After five through ten years	837,818
Over ten years	2,919,371
Amortized cost	4,029,655
Estimated Fair Value
Within one year or less	22,008
One through five years	250,806
After five through ten years	840,128
Over ten years	2,896,357
Estimated fair value	$ 4,009,299	$ 3,446,097
Weighted Average Yield
Within one year or less	2.65%
One through five years	2.75%
After five through ten years	3.24%
Over ten years	2.98%
Weighted average yield	3.01%
Amortized Cost
Within one year or less	$ 1,622
One through five years	10,234
After five through ten years	21,161
Over ten years	51,500
Amortized cost	84,517	89,216
Estimated Fair Value
Within one year or less	1,626
One through five years	10,378
After five through ten years	21,836
Over ten years	51,608
Estimated fair value	$ 85,448	$ 89,932