Loans Held for Investment and Allowance for Loan Losses - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual, past due 90 days or more and still accruing, restructured loans and foreclosed assets	$ 30,114,000	$ 39,724,000	$ 30,114,000	$ 39,724,000	$ 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	3,311,924,000		3,311,924,000
Unfunded loan commitments	6,751,000	809,000	6,751,000	809,000	$ 5,486,000
Provision for loan losses	(1,039,000)	$ 8,700,000	(4,468,000)	18,550,000
Provision for credit losses	(1,206,000)		(3,203,000)	$ 18,550,000
Unfunded Loan Commitment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for loan losses	$ 167,000		$ 1,265,000