Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Weighted Average Yield
Within one year or less	0.65%
One through five years	2.02%
After five through ten years	2.38%
Over ten years	2.28%
Weighted average yield	2.27%
Amortized Cost
Within one year or less	$ 38,171
One through five years	133,177
After five through ten years	840,457
Over ten years	3,743,299
Amortized cost	4,755,104
Estimated Fair Value
Within one year or less	38,071
One through five years	133,407
After five through ten years	842,821
Over ten years	3,722,040
Estimated fair value	$ 4,736,339	$ 3,446,097
Weighted Average Yield
Within one year or less	2.86%
One through five years	3.01%
After five through ten years	2.80%
Over ten years	2.59%
Weighted average yield	2.66%
Amortized Cost
Within one year or less	$ 1,629
One through five years	8,736
After five through ten years	35,075
Over ten years	130,466
Amortized cost	175,906	89,216
Estimated Fair Value
Within one year or less	1,642
One through five years	8,838
After five through ten years	35,733
Over ten years	130,102
Estimated fair value	$ 176,315	$ 89,932