Loans Held for Investment and Allowance for Credit Losses - Non-Accrual Loans, Loans Still Accruing and Past Due 90 Days or More and Restructured Loans (Parenthetical) (Detail) - USD ($)		Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	[1]	$ 39,226,000	$ 24,582,000	$ 28,508,000
Troubled debt restructured loans	[2]	26,000	26,000	472,000
Doubtful [Member]
Loans and Leases Receivable Disclosure [Line Items]
Troubled debt restructured loans		4,733,000	4,791,000	4,566,000
Purchased Credit Impaired Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans		$ 7,773,000	$ 251,000	$ 859,000

[1] Includes $7,773,000, $859,000 and $251,000 of purchased credit impaired loans as of March 31, 2020 and 2019, and December 31, 2019, respectively.

[2] Troubled debt restructured loans of $4,733,000, $4,566,000 and $4,791,000, whose interest collection, after considering economic and business conditions and collection efforts, is doubtful are included in non-accrual loans at March 31, 2020 and 2019, and December 31, 2019, respectively.