Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Debt securities available for sale:
Due in one year or less	$ 105,225
Due after one year through five years	625,911
Due after five years through ten years	3,968
Due after ten years	128,741
Total available for sale (amortized cost)	863,845
Mortgage-backed securities available for sale	10,617,657
Total	11,481,502
Debt securities held to maturity:
Due in one year or less	25,141
Due after one year through five years	43,474
Due after five years through ten years	3,643
Due after ten years	5,743
Total available for held to maturity (amortized cost)	78,001
Mortgage-backed securities held to maturity	2,331,949
Amortized cost for held to maturity	2,409,950	$ 2,859,709
Debt securities available for sale:
Due in one year or less	105,679
Due after one year through five years	627,585
Due after five years through ten years	4,538
Due after ten years	111,840
Total available for sale (fair value)	849,642
Mortgage-backed securities available for sale	10,930,713
Total	11,780,355
Debt securities held to maturity:
Due in one year or less	25,301
Due after one year through five years	43,474
Due after five years through ten years	3,721
Due after ten years	5,743
Total available for held to maturity (fair value)	78,239
Mortgage-backed securities held to maturity	2,377,858
Total	$ 2,456,097	$ 2,864,147