Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt securities available for sale:
Due in one year or less	$ 1,297,040
Due after one year through five years	436,548
Due after five years through ten years	96,521
Due after ten years	35,422
Total available for sale (amortized cost)	1,865,531
Mortgage-backed securities available for sale	8,046,629
Total	9,912,160
Debt securities held to maturity:
Due in one year or less	7,630
Due after one year through five years	5,917
Due after ten years	4,597
Total available for held to maturity (amortized cost)	18,144
Mortgage-backed securities held to maturity	3,082,951
Amortized cost for held to maturity	3,101,095	$ 3,353,213
Debt securities available for sale:
Due in one year or less	1,286,504
Due after one year through five years	428,617
Due after five years through ten years	95,992
Due after ten years	30,410
Total available for sale (fair value)	1,841,523
Mortgage-backed securities available for sale	7,817,323
Total	9,658,846
Debt securities held to maturity:
Due in one year or less	7,633
Due after one year through five years	5,951
Due after ten years	4,597
Total available for held to maturity (fair value)	18,181
Mortgage-backed securities held to maturity	3,004,148
Total	$ 3,022,329	$ 3,341,762