Loans Held for Investment and Allowance for Loan Losses - Non-Accrual Loans, Loans Still Accruing and Past Due 90 Days or More and Restructured Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Receivables [Abstract]
Non-accrual loans	$ 25,210	$ 42,619	$ 42,673
Loans still accruing and past due 90 days or more	23	113	23
Troubled debt restructured loans still accruing	22	24	25
Total	$ 25,255	$ 42,756	$ 42,721