Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Weighted Average Yield
Within one year or less	2.62%
One through five years	2.60%
After five through ten years	2.68%
Over ten years	2.59%
Weighted average yield	2.61%
Amortized Cost
Within one year or less	$ 41,572
One through five years	180,356
After five through ten years	641,404
Over ten years	2,927,043
Amortized Cost	3,790,375	$ 3,882,541
Estimated Fair Value
Within one year or less	41,999
One through five years	187,480
After five through ten years	671,014
Over ten years	3,014,467
Estimated fair value	$ 3,914,960	3,933,360
Weighted Average Yield
Within one year or less	3.00%
One through five years	2.59%
After five through ten years	2.37%
Over ten years	2.56%
Weighted average yield	2.50%
Amortized Cost
Within one year or less	$ 245
One through five years	5,329
After five through ten years	56,038
Over ten years	116,348
Amortized Cost	177,960	182,961
Estimated Fair Value
Within one year or less	245
One through five years	5,380
After five through ten years	58,524
Over ten years	122,247
Estimated fair value	$ 186,396	$ 189,899