Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt securities available for sale:
Due in one year or less	$ 616,908
Due after one year through five years	1,356,410
Due after five years through ten years	72,642
Due after ten years	59,165
Total available for sale (amortized cost)	2,105,125
Mortgage-backed securities available for sale	8,755,510
Total	10,860,635
Debt securities held to maturity:
Due in one year or less	14,622
Due after one year through five years	9,844
Due after five years through ten years	96
Due after ten years	5,010
Total available for held to maturity (amortized cost)	29,572
Mortgage-backed securities held to maturity	3,323,641
Amortized cost for held to maturity	3,353,213	$ 2,457,278
Debt securities available for sale:
Due in one year or less	613,866
Due after one year through five years	1,341,344
Due after five years through ten years	72,580
Due after ten years	51,118
Total available for sale (fair value)	2,078,908
Mortgage-backed securities available for sale	8,716,420
Total	10,795,328
Debt securities held to maturity:
Due in one year or less	14,656
Due after one year through five years	9,865
Due after five years through ten years	101
Due after ten years	5,010
Total available for held to maturity (fair value)	29,632
Mortgage-backed securities held to maturity	3,312,130
Total	$ 3,341,762	$ 2,451,222