Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Debt securities available for sale:
Due in one year or less	$ 157,954
Due after one year through five years	1,760,301
Due after five years through ten years	2,689
Due after ten years	128,841
Total available for sale (amortized cost)	2,049,785
Mortgage-backed securities available for sale	10,980,552
Total	13,030,337
Debt securities held to maturity:
Due in one year or less	24,533
Due after one year through five years	34,073
Due after five years through ten years	2,252
Due after ten years	5,543
Total available for held to maturity (amortized cost)	66,401
Mortgage-backed securities held to maturity	2,390,877
Amortized cost for held to maturity	2,457,278	$ 2,859,709
Debt securities available for sale:
Due in one year or less	158,334
Due after one year through five years	1,750,542
Due after five years through ten years	3,132
Due after ten years	112,693
Total available for sale (fair value)	2,024,701
Mortgage-backed securities available for sale	10,954,905
Total	12,979,606
Debt securities held to maturity:
Due in one year or less	24,643
Due after one year through five years	33,963
Due after five years through ten years	2,295
Due after ten years	5,543
Total available for held to maturity (fair value)	66,444
Mortgage-backed securities held to maturity	2,384,778
Total	$ 2,451,222	$ 2,864,147