Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Weighted Average Yield
Within one year or less	2.06%
One through five years	2.04%
After five through ten years	2.36%
Over ten years	2.38%
Weighted average yield	2.38%
Amortized Cost
Within one year or less	$ 6,667
One through five years	134,449
After five through ten years	887,796
Over ten years	3,639,113
Amortized cost	4,668,025
Estimated Fair Value
Within one year or less	6,670
One through five years	132,594
After five through ten years	868,090
Over ten years	3,535,132
Estimated fair value	$ 4,542,486	$ 4,590,062
Weighted Average Yield
Within one year or less	3.04%
One through five years	2.98%
After five through ten years	2.66%
Over ten years	2.56%
Weighted average yield	2.60%
Amortized Cost
Within one year or less	$ 1,445
One through five years	8,369
After five through ten years	45,690
Over ten years	168,737
Amortized cost	224,241	227,318
Estimated Fair Value
Within one year or less	1,448
One through five years	8,370
After five through ten years	45,450
Over ten years	164,600
Estimated fair value	$ 219,868	$ 227,964