Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Weighted Average Yield
Within one year or less	3.86%
One through five years	2.66%
After five through ten years	2.71%
Over ten years	2.97%
Weighted average yield	2.92%
Amortized Cost
Within one year or less	$ 2,536
One through five years	113,924
After five through ten years	863,166
Over ten years	3,901,803
Amortized Cost	4,881,429	$ 4,843,313
Estimated Fair Value
Within one year or less	2,542
One through five years	114,533
After five through ten years	867,609
Over ten years	3,889,094
Estimated fair value	$ 4,873,778	4,783,579
Weighted Average Yield
Within one year or less	2.33%
One through five years	2.42%
After five through ten years	2.58%
Over ten years	2.63%
Weighted average yield	2.61%
Amortized Cost
Within one year or less	$ 955
One through five years	6,100
After five through ten years	42,885
Over ten years	149,018
Amortized Cost	198,958	207,446
Estimated Fair Value
Within one year or less	959
One through five years	6,114
After five through ten years	43,419
Over ten years	150,076
Estimated fair value	$ 200,568	$ 204,277