Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale, Amortized Cost
Within one year	$ 353,414
One to five years	2,828,719
Five to ten years	1,029,340
Beyond ten years	1,573,281
Total	5,784,754	$ 5,313,209
Available-for-sale, Fair Value
Within one year	358,852
One to five years	2,922,695
Five to ten years	1,049,684
Beyond ten years	1,638,884
Total	$ 5,970,115	$ 5,385,091
Available-for-sale, Weighted Average Yield
Within one year	2.61%
One to five years	2.27%
Five to ten years	1.98%
Beyond ten years	2.90%
Total	2.41%