Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt securities available for sale:
Due in one year or less	$ 5,197
Due after one year through five years	11,446
Due after five years through ten years	98,426
Due after ten years	30,000
Total available for sale (amortized cost)	145,069
Mortgage-backed securities available for sale	4,944,995
Investment securities available for sale, amortized cost	5,090,064	$ 6,258,276
Debt securities held to maturity:
Due in one year or less	4,680
Due after one year through five years	175
Due after ten years	2,962
Total available for held to maturity (amortized cost)	7,817
Mortgage-backed securities held to maturity	1,930,877
Amortized cost for held to maturity	1,938,694	2,656,917
Debt securities available for sale:
Due in one year or less	5,207
Due after one year through five years	12,009
Due after five years through ten years	95,033
Due after ten years	24,372
Total available for sale (fair value)	136,621
Mortgage-backed securities available for sale	5,165,717
Total	5,302,338	6,318,776
Debt securities held to maturity:
Due in one year or less	4,688
Due after one year through five years	177
Due after ten years	2,962
Total available for held to maturity (fair value)	7,827
Mortgage-backed securities held to maturity	2,024,099
Total	$ 2,031,926	$ 2,699,206