Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-for-sale, Amortized Cost
Within one year	$ 479,460
One to five years	2,808,889
Five to ten years	750,252
Beyond ten years	1,375,376
Total	5,413,977	$ 5,313,209
Available-for-sale, Fair Value
Within one year	488,426
One to five years	2,909,519
Five to ten years	772,598
Beyond ten years	1,430,991
Total	$ 5,601,534	$ 5,385,091
Available-for-sale, Weighted Average Yield
Within one year	2.49%
One to five years	2.38%
Five to ten years	2.50%
Beyond ten years	3.05%
Total	2.58%