Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Weighted Average Yield
Within one year or less	2.34%
One through five years	1.75%
After five through ten years	2.36%
Over ten years	2.16%
Weighted average yield	2.15%
Amortized Cost
Within one year or less	$ 10,661
One through five years	309,881
After five through ten years	494,729
Over ten years	1,904,345
Amortized Cost	2,719,616
Estimated Fair Value
Within one year or less	10,751
One through five years	314,811
After five through ten years	507,477
Over ten years	1,922,386
Estimated fair value	$ 2,755,425	$ 2,800,286
Weighted Average Yield
Within one year or less	2.07%
One through five years	3.16%
After five through ten years	2.89%
Over ten years	2.96%
Weighted average yield	2.96%
Amortized Cost
Within one year or less	$ 759
One through five years	11,876
After five through ten years	18,389
Over ten years	65,093
Amortized cost	96,117	98,928
Estimated Fair Value
Within one year or less	768
One through five years	12,222
After five through ten years	19,246
Over ten years	66,803
Estimated fair value	$ 99,039	$ 100,961