Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail)	Dec. 31, 2020 USD ($) Investment	Dec. 31, 2019 USD ($) Investment
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 4,285,435,000	$ 2,102,365,000
Less than 12 months - Unrealized Losses	(14,988,000)	(20,007,000)
12 months or longer - Fair Value of Investments	0	449,850,000
12 months or longer - Unrealized Losses	0	(135,000)
Fair Value of Investments	4,285,435,000	2,552,215,000
Unrealized Losses	$ (14,988,000)	$ (20,142,000)
Number of investments in unrealized loss position | Investment	93	58
Number of investments with unrealized losses greater than 12 months | Investment	0	12
Investments in our AFS securities portfolio past due	$ 0
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	59,929,000	$ 971,572,000
Less than 12 months - Unrealized Losses	(107,000)	(3,996,000)
12 months or longer - Fair Value of Investments	0	449,850,000
12 months or longer - Unrealized Losses	0	(135,000)
Fair Value of Investments	59,929,000	1,421,422,000
Unrealized Losses	(107,000)	(4,131,000)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	133,143,000	99,547,000
Less than 12 months - Unrealized Losses	(585,000)	(453,000)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	133,143,000	99,547,000
Unrealized Losses	(585,000)	(453,000)
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	903,767,000	4,014,000
Less than 12 months - Unrealized Losses	(651,000)	(19,000)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	903,767,000	4,014,000
Unrealized Losses	(651,000)	(19,000)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations - fixed rate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	2,199,207,000
Less than 12 months - Unrealized Losses	(10,278,000)
12 months or longer - Fair Value of Investments	0
12 months or longer - Unrealized Losses	0
Fair Value of Investments	2,199,207,000
Unrealized Losses	(10,278,000)
Agency-issued commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	989,389,000	1,027,232,000
Less than 12 months - Unrealized Losses	(3,367,000)	(15,539,000)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	989,389,000	1,027,232,000
Unrealized Losses	$ (3,367,000)	$ (15,539,000)