Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Weighted Average Yield
Within one year or less	1.77%
One through five years	2.24%
After five through ten years	2.42%
Over ten years	2.43%
Weighted average yield	2.42%
Amortized Cost
Within one year or less	$ 18,264
One through five years	132,130
After five through ten years	1,034,731
Over ten years	3,612,515
Amortized cost	4,797,640
Estimated Fair Value
Within one year or less	18,140
One through five years	129,928
After five through ten years	1,007,737
Over ten years	3,495,110
Estimated fair value	$ 4,650,915	$ 4,590,062
Weighted Average Yield
Within one year or less	4.00%
One through five years	2.98%
After five through ten years	2.60%
Over ten years	2.63%
Weighted average yield	2.64%
Amortized Cost
Within one year or less	$ 1,436
One through five years	8,350
After five through ten years	44,076
Over ten years	167,168
Amortized cost	221,030	227,318
Estimated Fair Value
Within one year or less	1,437
One through five years	8,329
After five through ten years	43,775
Over ten years	162,768
Estimated fair value	$ 216,309	$ 227,964