Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt securities available for sale:
Due in one year or less	$ 4,291
Due after one year through five years	13,974
Due after five years through ten years	97,578
Due after ten years	30,000
Total available for sale (amortized cost)	145,843
Mortgage-backed securities available for sale	3,328,080
Investment securities available for sale, amortized cost	3,473,923	$ 4,621,027
Debt securities held to maturity:
Due in one year or less	176
Due after one year through five years	3,066
Due after ten years	2,622
Total available for held to maturity (amortized cost)	5,864
Mortgage-backed securities held to maturity	2,353,863
Amortized cost for held to maturity	2,359,727	1,748,989
Debt securities available for sale:
Due in one year or less	4,351
Due after one year through five years	14,616
Due after five years through ten years	98,981
Due after ten years	24,679
Total available for sale (fair value)	142,627
Mortgage-backed securities available for sale	3,475,479
Total	3,618,106	4,822,606
Debt securities held to maturity:
Due in one year or less	177
Due after one year through five years	3,065
Due after ten years	2,622
Total available for held to maturity (fair value)	5,864
Mortgage-backed securities held to maturity	2,408,074
Total	$ 2,413,938	$ 1,842,281