Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Weighted Average Yield
Within one year or less	2.36%
One through five years	1.64%
After five through ten years	2.29%
Over ten years	1.97%
Weighted average yield	2.01%
Amortized Cost
Within one year or less	$ 19,560
One through five years	275,596
After five through ten years	585,382
Over ten years	1,965,431
Amortized Cost	2,845,969
Estimated Fair Value
Within one year or less	19,658
One through five years	279,271
After five through ten years	602,194
Over ten years	1,984,290
Estimated fair value	$ 2,885,413	$ 2,800,286
Weighted Average Yield
Within one year or less	2.42%
One through five years	2.97%
After five through ten years	3.07%
Over ten years	2.74%
Weighted average yield	2.82%
Amortized Cost
Within one year or less	$ 2,059
One through five years	10,295
After five through ten years	18,357
Over ten years	59,942
Amortized cost	90,653	98,928
Estimated Fair Value
Within one year or less	2,076
One through five years	10,583
After five through ten years	19,214
Over ten years	61,537
Estimated fair value	$ 93,410	$ 100,961