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 (Parenthetical) (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	$ 25,587	$ 17,670	$ 18,750
Troubled debt restructured loans	513	627	668
Doubtful [Member]
Loans and Leases Receivable Disclosure [Line Items]
Troubled debt restructured loans	4,577	4,629	5,277
Purchased Credit Impaired Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Non-accrual loans	$ 2,947	$ 618	$ 736