Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Debt securities available for sale:
Due in one year or less	$ 6,147
Due after one year through five years	399,312
Due after five years through ten years	3,425
Due after ten years	157,524
Total available for sale (amortized cost)	566,408
Mortgage-backed securities available for sale	10,944,734
Total	11,511,142
Debt securities held to maturity:
Due in one year or less	28,259
Due after one year through five years	51,414
Due after five years through ten years	4,941
Due after ten years	6,011
Total available for held to maturity (amortized cost)	90,625
Mortgage-backed securities held to maturity	2,483,796
Amortized cost for held to maturity	2,574,421	$ 2,859,709
Debt securities available for sale:
Due in one year or less	6,176
Due after one year through five years	402,100
Due after five years through ten years	3,822
Due after ten years	151,567
Total available for sale (fair value)	563,665
Mortgage-backed securities available for sale	11,267,335
Total	11,831,000
Debt securities held to maturity:
Due in one year or less	28,430
Due after one year through five years	51,611
Due after five years through ten years	5,045
Due after ten years	6,011
Total available for held to maturity (fair value)	91,097
Mortgage-backed securities held to maturity	2,532,162
Total	$ 2,623,259	$ 2,864,147