Loans - Summary of Recorded Investment of Purchased Impaired Loans (Detail) - PCI Loans - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
30-89 Days Past Due	$ 32	$ 510
90 Days or More Past Due and Still Accruing	129	165
Nonaccruals	0	0
Total Past Due and Nonaccruals	161	675
Current	5,295	5,150
Total PCI Loans	5,456	5,825
Construction, land and land development
Financing Receivable, Impaired [Line Items]
Nonaccruals	0	0
Current	1,385	1,405
Total PCI Loans	1,385	1,405
Commercial mortgages (non-owner occupied)
Financing Receivable, Impaired [Line Items]
Nonaccruals	0	0
Current	149	171
Total PCI Loans	149	171
Commercial mortgages (owner occupied)
Financing Receivable, Impaired [Line Items]
30-89 Days Past Due	32	161
90 Days or More Past Due and Still Accruing	28
Nonaccruals	0	0
Total Past Due and Nonaccruals	60	161
Current	238	160
Total PCI Loans	298	321
Residential first mortgages
Financing Receivable, Impaired [Line Items]
30-89 Days Past Due		349
90 Days or More Past Due and Still Accruing	78	141
Nonaccruals	0	0
Total Past Due and Nonaccruals	78	490
Current	3,448	3,320
Total PCI Loans	3,526	3,810
Residential revolving and junior mortgages
Financing Receivable, Impaired [Line Items]
90 Days or More Past Due and Still Accruing	20	20
Nonaccruals	0	0
Total Past Due and Nonaccruals	20	20
Current	21	29
Total PCI Loans	41	49
Commercial and industrial
Financing Receivable, Impaired [Line Items]
30-89 Days Past Due	0	0
90 Days or More Past Due and Still Accruing	0	0
Nonaccruals	0	0
Total Past Due and Nonaccruals	0	0
Current	0	0
Total PCI Loans	0	0
Consumer
Financing Receivable, Impaired [Line Items]
90 Days or More Past Due and Still Accruing	3	4
Nonaccruals	0	0
Total Past Due and Nonaccruals	3	4
Current	54	65
Total PCI Loans	$ 57	$ 69