Loans Held for Investment and Allowance for Loan Losses - Schedule of Impaired Loans and Related Allowance (Parenthetical) (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance	$ 7,732,000	$ 3,009,000	$ 4,621,000
Purchased Credit Impaired Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance	$ 2,947,000	$ 618,000	$ 736,000

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant