Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Weighted Average Yield
Within one year or less	2.55%
One through five years	2.62%
After five through ten years	2.76%
Over ten years	2.67%
Weighted average yield	2.68%
Amortized Cost
Within one year or less	$ 10,720
One through five years	123,447
After five through ten years	760,325
Over ten years	3,281,041
Amortized Cost	4,175,533	$ 4,843,313
Estimated Fair Value
Within one year or less	10,787
One through five years	125,721
After five through ten years	786,040
Over ten years	3,315,534
Estimated fair value	$ 4,238,082	4,783,579
Weighted Average Yield
Within one year or less	2.33%
One through five years	2.63%
After five through ten years	2.35%
Over ten years	2.58%
Weighted average yield	2.53%
Amortized Cost
Within one year or less	$ 948
One through five years	4,483
After five through ten years	43,085
Over ten years	136,491
Amortized Cost	185,007	207,446
Estimated Fair Value
Within one year or less	949
One through five years	4,518
After five through ten years	44,653
Over ten years	142,466
Estimated fair value	$ 192,586	$ 204,277