Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Debt securities available for sale:
Due in one year or less	$ 8,282
Due after one year through five years	197,796
Due after five years through ten years	3,296
Due after ten years	159,824
Total available for sale (amortized cost)	369,198
Mortgage-backed securities available for sale	10,504,838
Total	10,874,036
Debt securities held to maturity:
Due in one year or less	30,523
Due after one year through five years	74,511
Due after five years through ten years	20,217
Due after ten years	6,806
Total available for held to maturity (amortized cost)	132,057
Mortgage-backed securities held to maturity	2,866,429
Amortized cost for held to maturity	2,998,486	$ 3,507,868
Debt securities available for sale:
Due in one year or less	8,330
Due after one year through five years	199,712
Due after five years through ten years	3,495
Due after ten years	164,497
Total available for sale (fair value)	376,034
Mortgage-backed securities available for sale	10,714,130
Total	11,090,164
Debt securities held to maturity:
Due in one year or less	30,706
Due after one year through five years	75,150
Due after five years through ten years	20,443
Due after ten years	6,806
Total available for held to maturity (fair value)	133,105
Mortgage-backed securities held to maturity	2,892,582
Total	$ 3,025,687	$ 3,538,282