Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt securities available for sale:
Due in one year or less	$ 1,344,408
Due after one year through five years	9,555
Due after five years through ten years	102,081
Due after ten years	30,015
Total available for sale (amortized cost)	1,486,059
Mortgage-backed securities available for sale	7,383,364
Investment securities available for sale, amortized cost	8,869,423	$ 10,938,796
Debt securities held to maturity:
Due in one year or less	449,468
Due after one year through five years	4,568
Due after ten years	3,668
Total available for held to maturity (amortized cost)	457,704
Mortgage-backed securities held to maturity	2,858,936
Amortized cost for held to maturity	3,316,640	3,353,213
Debt securities available for sale:
Due in one year or less	1,334,695
Due after one year through five years	9,319
Due after five years through ten years	95,653
Due after ten years	25,829
Total available for sale (fair value)	1,465,496
Mortgage-backed securities available for sale	7,217,013
Total	8,682,509	10,896,284
Debt securities held to maturity:
Due in one year or less	449,237
Due after one year through five years	4,570
Due after ten years	3,668
Total available for held to maturity (fair value)	457,475
Mortgage-backed securities held to maturity	2,798,008
Total	$ 3,255,483	$ 3,341,762