Loans Held for Investment and Allowance for Loan Losses - Schedule of Impaired Loans and Related Allowance (Parenthetical) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019		Dec. 31, 2018		Jun. 30, 2018
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance	$ 11,707	[1]	$ 13,741	[2]	$ 7,052	[3]
Purchased Credit Impaired Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance	$ 464		$ 827		$ 2,813

[1] Includes $464,000 of purchased credit impaired loans.

[2] Includes $827,000 of purchased credit impaired loans.

[3] Includes $2,813,000 of purchased credit impaired loans.