Loans Held for Investment and Allowance for Loan Losses - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) Restructuredloan	Sep. 30, 2020 USD ($) Restructuredloan	Sep. 30, 2021 USD ($) Restructuredloan	Sep. 30, 2020 USD ($) Restructuredloan	Dec. 31, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual, past due 90 days or more and still accruing, restructured loans and foreclosed assets	$ 25,283,000	$ 43,052,000	$ 25,283,000	$ 43,052,000	$ 42,898,000
Additional funds advanced in connection with impaired loans	0		$ 0
Default for purposes of this disclosure is a troubled debt restructured loan			90 days
Commitments to lend additional funds to borrowers with loan that have been modified as TDRs	0		$ 0
Loans held by subsidiaries subject to blanket liens	3,434,685,000		3,434,685,000
Line of credit	$ 0	$ 0	$ 0	$ 0	0
Troubled debt restructured loan | Restructuredloan	1	0	1	0
Unfunded loan commitments	$ 6,751,000	$ 2,309,000,000	$ 6,751,000	$ 2,309,000,000	$ 5,486,000
Provision for Loan and Lease Losses	0	7,500,000	(4,468,000)	26,050,000
Provision for credit losses		(9,000,000)	(3,203,000)	27,550,000
Federal Home Loan Bank Borrowings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Available line of credit	1,643,084,000,000		1,643,084,000,000
Line of credit	$ 0		0
Unfunded Loan Commitment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for Loan and Lease Losses		$ 1,500,000	$ 1,265,000	$ 1,500,000