Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt securities available for sale:
Due in one year or less	$ 4,418
Due after one year through five years	10,717
Due after five years through ten years	99,212
Due after ten years	30,000
Total available for sale (amortized cost)	144,347
Mortgage-backed securities available for sale	6,113,929
Investment securities available for sale, amortized cost	6,258,276	$ 8,869,423
Debt securities held to maturity:
Due in one year or less	251,457
Due after one year through five years	2,545
Due after ten years	3,239
Total available for held to maturity (amortized cost)	257,241
Mortgage-backed securities held to maturity	2,399,676
Amortized cost for held to maturity	2,656,917	3,316,640
Debt securities available for sale:
Due in one year or less	4,427
Due after one year through five years	10,777
Due after five years through ten years	98,616
Due after ten years	24,000
Total available for sale (fair value)	137,820
Mortgage-backed securities available for sale	6,180,956
Total	6,318,776	8,682,509
Debt securities held to maturity:
Due in one year or less	251,749
Due after one year through five years	2,555
Due after ten years	3,239
Total available for held to maturity (fair value)	257,543
Mortgage-backed securities held to maturity	2,441,663
Total	$ 2,699,206	$ 3,255,483