Allowance for Loan Losses - Summary of Impaired Loans Evaluated for Specific Reserves (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Total real estate loans | New Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	$ 66	$ 66
Impaired loans - With allowance, Unpaid principal balance	66	66
Impaired loans - With allowance, Related allowance	66	66
Impaired loans - With no allowance, Recorded investment	587	1,159
Impaired loans - With no allowance, Unpaid principal balance	587	1,159
Total real estate loans | Acquired Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	336	336
Impaired loans - With allowance, Unpaid principal balance	347	347
Impaired loans - With allowance, Related allowance	129	129
Impaired loans - With no allowance, Recorded investment	4,273	4,319
Impaired loans - With no allowance, Unpaid principal balance	5,753	5,753
Commercial real estate | New Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	0	0
Impaired loans - With allowance, Unpaid principal balance	0	0
Impaired loans - With allowance, Related allowance	0	0
Impaired loans - With no allowance, Recorded investment	0	0
Impaired loans - With no allowance, Unpaid principal balance	0	0
Commercial real estate | Acquired Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	336	336
Impaired loans - With allowance, Unpaid principal balance	347	347
Impaired loans - With allowance, Related allowance	129	129
Impaired loans - With no allowance, Recorded investment	3,511	3,557
Impaired loans - With no allowance, Unpaid principal balance	4,991	4,991
Owner-occupied commercial real estate | New Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	0	0
Impaired loans - With allowance, Unpaid principal balance	0	0
Impaired loans - With allowance, Related allowance	0	0
Impaired loans - With no allowance, Recorded investment	0	0
Impaired loans - With no allowance, Unpaid principal balance	0	0
Owner-occupied commercial real estate | Acquired Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	0	0
Impaired loans - With allowance, Unpaid principal balance	0	0
Impaired loans - With allowance, Related allowance	0	0
Impaired loans - With no allowance, Recorded investment	0	0
Impaired loans - With no allowance, Unpaid principal balance	0	0
1-4 single family residential | New Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	0	0
Impaired loans - With allowance, Unpaid principal balance	0	0
Impaired loans - With allowance, Related allowance	0	0
Impaired loans - With no allowance, Recorded investment	524	1,096
Impaired loans - With no allowance, Unpaid principal balance	524	1,096
1-4 single family residential | Acquired Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	0	0
Impaired loans - With allowance, Unpaid principal balance	0	0
Impaired loans - With allowance, Related allowance	0	0
Impaired loans - With no allowance, Recorded investment	267	267
Impaired loans - With no allowance, Unpaid principal balance	267	267
Construction, land and development | New Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	0	0
Impaired loans - With allowance, Unpaid principal balance	0	0
Impaired loans - With allowance, Related allowance	0	0
Impaired loans - With no allowance, Recorded investment	0	0
Impaired loans - With no allowance, Unpaid principal balance	0	0
Construction, land and development | Acquired Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	0	0
Impaired loans - With allowance, Unpaid principal balance	0	0
Impaired loans - With allowance, Related allowance	0	0
Impaired loans - With no allowance, Recorded investment	0	0
Impaired loans - With no allowance, Unpaid principal balance	0	0
Home equity loans and lines of credit | New Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	66	66
Impaired loans - With allowance, Unpaid principal balance	66	66
Impaired loans - With allowance, Related allowance	66	66
Impaired loans - With no allowance, Recorded investment	63	63
Impaired loans - With no allowance, Unpaid principal balance	63	63
Home equity loans and lines of credit | Acquired Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	0	0
Impaired loans - With allowance, Unpaid principal balance	0	0
Impaired loans - With allowance, Related allowance	0	0
Impaired loans - With no allowance, Recorded investment	495	495
Impaired loans - With no allowance, Unpaid principal balance	495	495
Total other loans | New Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	0	0
Impaired loans - With allowance, Unpaid principal balance	0	0
Impaired loans - With allowance, Related allowance	0	0
Impaired loans - With no allowance, Recorded investment	0	0
Impaired loans - With no allowance, Unpaid principal balance	0	0
Total other loans | Acquired Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	272	272
Impaired loans - With allowance, Unpaid principal balance	272	272
Impaired loans - With allowance, Related allowance	272	272
Impaired loans - With no allowance, Recorded investment	0	0
Impaired loans - With no allowance, Unpaid principal balance	0	0
Commercial and industrial | New Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	0	0
Impaired loans - With allowance, Unpaid principal balance	0	0
Impaired loans - With allowance, Related allowance	0	0
Impaired loans - With no allowance, Recorded investment	0	0
Impaired loans - With no allowance, Unpaid principal balance	0	0
Commercial and industrial | Acquired Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	272	272
Impaired loans - With allowance, Unpaid principal balance	272	272
Impaired loans - With allowance, Related allowance	272	272
Impaired loans - With no allowance, Recorded investment	0	0
Impaired loans - With no allowance, Unpaid principal balance	0	0
Consumer | New Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	0	0
Impaired loans - With allowance, Unpaid principal balance	0	0
Impaired loans - With allowance, Related allowance	0	0
Impaired loans - With no allowance, Recorded investment	0	0
Impaired loans - With no allowance, Unpaid principal balance	0	0
Consumer | Acquired Loans
Financing Receivable, Impaired [Line Items]
Impaired loans - With allowance, Recorded investment	0	0
Impaired loans - With allowance, Unpaid principal balance	0	0
Impaired loans - With allowance, Related allowance	0	0
Impaired loans - With no allowance, Recorded investment	0	0
Impaired loans - With no allowance, Unpaid principal balance	$ 0	$ 0