Securities - Summary Of Government-Sponsored Residential Mortgage-Backed Securities With Gross Unrealized Losses (Detail) $ in Thousands	Mar. 31, 2021 USD ($) loans	Dec. 31, 2020 USD ($)
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Number of holdings	40	6
Less than 12 Months, Gross Unrealized Loss	$ 77,395	$ 3,872
Less than 12 Months, Fair Value	3,011,821	416,824
12 Months or Longer, Gross Unrealized Loss	0	0
12 Months or Longer, Fair Value	0	0
Gross Unrealized Losses	77,395	3,872
Fair Value	$ 3,011,821	$ 416,824
Government-sponsored residential mortgage-backed securities
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Number of holdings	18
Less than 12 Months, Gross Unrealized Loss	$ 45,395
Less than 12 Months, Fair Value	2,004,883
12 Months or Longer, Gross Unrealized Loss	0
12 Months or Longer, Fair Value	0
Gross Unrealized Losses	45,395
Fair Value	$ 2,004,883
Government-sponsored commercial mortgage-backed securities
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Number of holdings	7
Less than 12 Months, Gross Unrealized Loss	$ 2,793
Less than 12 Months, Fair Value	116,271
12 Months or Longer, Gross Unrealized Loss	0
12 Months or Longer, Fair Value	0
Gross Unrealized Losses	2,793
Fair Value	$ 116,271
U.S. Agency bonds
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Number of holdings	13	6
Less than 12 Months, Gross Unrealized Loss	$ 28,882	$ 3,872
Less than 12 Months, Fair Value	831,672	416,824
12 Months or Longer, Gross Unrealized Loss	0	0
12 Months or Longer, Fair Value	0	0
Gross Unrealized Losses	28,882	3,872
Fair Value	$ 831,672	$ 416,824
U.S. Treasury securities
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Number of holdings | loans	2
Less than 12 Months, Gross Unrealized Loss	$ 325
Less than 12 Months, Fair Value	58,995
12 Months or Longer, Gross Unrealized Loss	0
12 Months or Longer, Fair Value	0
Gross Unrealized Losses	325
Fair Value	$ 58,995