Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Weighted Average Yield
Within one year or less	2.36%
One through five years	1.63%
After five through ten years	2.31%
Over ten years	2.10%
Weighted average yield	2.10%
Amortized Cost
Within one year or less	$ 12,975
One through five years	279,112
After five through ten years	556,517
Over ten years	1,878,211
Amortized Cost	2,726,815
Estimated Fair Value
Within one year or less	13,093
One through five years	283,824
After five through ten years	574,992
Over ten years	1,904,106
Estimated fair value	$ 2,776,015	$ 2,800,286
Weighted Average Yield
Within one year or less	2.63%
One through five years	2.87%
After five through ten years	3.02%
Over ten years	3.01%
Weighted average yield	2.99%
Amortized Cost
Within one year or less	$ 1,026
One through five years	12,146
After five through ten years	16,603
Over ten years	63,129
Amortized cost	92,904	98,928
Estimated Fair Value
Within one year or less	1,036
One through five years	12,504
After five through ten years	17,511
Over ten years	65,188
Estimated fair value	$ 96,239	$ 100,961