Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt securities available for sale:
Due in one year or less	$ 817,981
Due after one year through five years	1,151,292
Due after five years through ten years	82,301
Due after ten years	49,543
Total available for sale (amortized cost)	2,101,117
Mortgage-backed securities available for sale	8,407,503
Total	10,508,620
Debt securities held to maturity:
Due in one year or less	11,410
Due after one year through five years	8,703
Due after five years through ten years	96
Due after ten years	4,720
Total available for held to maturity (amortized cost)	24,929
Mortgage-backed securities held to maturity	3,203,827
Amortized cost for held to maturity	3,228,756	$ 3,353,213
Debt securities available for sale:
Due in one year or less	813,574
Due after one year through five years	1,134,986
Due after five years through ten years	82,049
Due after ten years	45,091
Total available for sale (fair value)	2,075,700
Mortgage-backed securities available for sale	8,222,156
Total	10,297,856
Debt securities held to maturity:
Due in one year or less	11,432
Due after one year through five years	8,716
Due after five years through ten years	100
Due after ten years	4,720
Total available for held to maturity (fair value)	24,968
Mortgage-backed securities held to maturity	3,135,247
Total	$ 3,160,215	$ 3,341,762