Loans - Allowances for Loan Losses and Ending Balances by Portfolio Class and Based on Impairment Method (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	$ 49		$ 729
Collectively evaluated for impairment	5,387		4,562
Total ending allowance balance	5,436	$ 6,289	5,291	$ 5,002	$ 5,156	$ 4,662
Loans:
Loans individually evaluated for impairment	6,430		7,156
Loans collectively evaluated for impairment	502,499		467,812
Total ending loans balance	508,929		474,968
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	5		705
Collectively evaluated for impairment	2,104		1,502
Total ending allowance balance	2,109	2,362	2,207	2,219	2,376	1,664
Loans:
Loans individually evaluated for impairment	1,241		5,028
Loans collectively evaluated for impairment	136,218		129,240
Total ending loans balance	137,459		134,268
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	20
Collectively evaluated for impairment	1,610		1,264
Total ending allowance balance	1,630	1,718	1,264	1,287	1,262	1,271
Loans:
Loans individually evaluated for impairment	3,703		621
Loans collectively evaluated for impairment	173,885		158,854
Total ending loans balance	177,588		159,475
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	24		24
Collectively evaluated for impairment	1,187		1,165
Total ending allowance balance	1,211	1,264	1,189	1,146	1,095	1,086
Loans:
Loans individually evaluated for impairment	1,486		1,507
Loans collectively evaluated for impairment	151,795		142,982
Total ending loans balance	153,281		144,489
Construction & Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	243		178
Total ending allowance balance	243	222	178	151	127	123
Loans:
Loans collectively evaluated for impairment	23,915		23,428
Total ending loans balance	23,915		23,428
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	178		141
Total ending allowance balance	178	182	141	137	110	86
Loans:
Loans collectively evaluated for impairment	16,686		13,308
Total ending loans balance	16,686		13,308
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	65		312
Total ending allowance balance	$ 65	$ 541	$ 312	$ 62	$ 186	$ 432