Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Debt securities available for sale:
Due in one year or less	$ 106,349
Due after one year through five years	202,864
Due after five years through ten years	2,729
Due after ten years	159,866
Total available for sale (amortized cost)	471,808
Mortgage-backed securities available for sale	11,593,033
Total	12,064,841
Debt securities held to maturity:
Due in one year or less	34,174
Due after one year through five years	72,120
Due after five years through ten years	12,137
Due after ten years	6,575
Total available for held to maturity (amortized cost)	125,006
Mortgage-backed securities held to maturity	2,734,703
Amortized cost for held to maturity	2,859,709	$ 3,507,868
Debt securities available for sale:
Due in one year or less	106,383
Due after one year through five years	203,235
Due after five years through ten years	2,941
Due after ten years	159,739
Total available for sale (fair value)	472,298
Mortgage-backed securities available for sale	11,686,702
Total	12,159,000
Debt securities held to maturity:
Due in one year or less	34,311
Due after one year through five years	72,419
Due after five years through ten years	12,283
Due after ten years	6,575
Total available for held to maturity (fair value)	125,588
Mortgage-backed securities held to maturity	2,738,559
Total	$ 2,864,147	$ 3,538,282