Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Weighted Average Yield
Within one year or less	3.67%
One through five years	2.66%
After five through ten years	2.72%
Over ten years	2.84%
Weighted average yield	2.82%
Amortized Cost
Within one year or less	$ 1,554
One through five years	112,248
After five through ten years	769,216
Over ten years	3,518,983
Amortized Cost	4,402,001	$ 4,843,313
Estimated Fair Value
Within one year or less	1,558
One through five years	113,915
After five through ten years	789,351
Over ten years	3,550,484
Estimated fair value	$ 4,455,308	4,783,579
Weighted Average Yield
Within one year or less	2.34%
One through five years	2.63%
After five through ten years	2.43%
Over ten years	2.60%
Weighted average yield	2.56%
Amortized Cost
Within one year or less	$ 952
One through five years	4,484
After five through ten years	45,338
Over ten years	142,143
Amortized Cost	192,917	207,446
Estimated Fair Value
Within one year or less	955
One through five years	4,524
After five through ten years	46,508
Over ten years	146,025
Estimated fair value	$ 198,012	$ 204,277