Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Weighted Average Yield
Within one year or less	1.63%
One through five years	1.69%
After five through ten years	2.23%
Over ten years	2.12%
Weighted average yield	2.09%
Amortized Cost
Within one year or less	$ 25,724
One through five years	298,322
After five through ten years	516,467
Over ten years	1,961,238
Amortized Cost	2,801,751
Estimated Fair Value
Within one year or less	25,824
One through five years	301,724
After five through ten years	524,969
Over ten years	1,975,288
Estimated fair value	$ 2,827,805	$ 2,158,853
Weighted Average Yield
Within one year or less	3.85%
One through five years	2.79%
After five through ten years	3.09%
Over ten years	2.98%
Weighted average yield	2.98%
Amortized Cost
Within one year or less	$ 75
One through five years	11,552
After five through ten years	20,663
Over ten years	66,040
Amortized cost	98,330	116,960
Estimated Fair Value
Within one year or less	75
One through five years	11,808
After five through ten years	21,494
Over ten years	67,234
Estimated fair value	$ 100,611	$ 119,481