Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Weighted Average Yield
Within one year or less	2.61%
One through five years	2.52%
After five through ten years	2.72%
Over ten years	2.45%
Weighted average yield	2.50%
Amortized Cost
Within one year or less	$ 40,875
One through five years	127,588
After five through ten years	706,365
Over ten years	3,007,713
Amortized Cost	3,882,541	$ 4,843,313
Estimated Fair Value
Within one year or less	41,158
One through five years	130,809
After five through ten years	725,592
Over ten years	3,035,801
Estimated fair value	$ 3,933,360	4,783,579
Weighted Average Yield
Within one year or less	3.29%
One through five years	2.60%
After five through ten years	2.37%
Over ten years	2.57%
Weighted average yield	2.53%
Amortized Cost
Within one year or less	$ 330
One through five years	5,350
After five through ten years	42,775
Over ten years	134,506
Amortized Cost	182,961	207,446
Estimated Fair Value
Within one year or less	331
One through five years	5,380
After five through ten years	44,314
Over ten years	139,874
Estimated fair value	$ 189,899	$ 204,277