Allowance for Loan Losses - Loans Evaluated for Impairment Individually and Collectively by Segment (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans individually evaluated for impairment	$ 1,256		$ 1,094
Loans collectively evaluated for impairment	5,857		6,676
Purchased credit impaired loans	0		0
Total allowance on loan losses	7,113	$ 7,923	7,770	$ 4,241	$ 3,993	$ 3,863
Loans individually evaluated for impairment	8,168		9,107
Loans collectively evaluated for impairment	787,726		751,364
Purchased credit impaired loans	5,638		5,825
Total Loans	801,532		766,296
Mortgage Loans on Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans individually evaluated for impairment	1,125		861
Loans collectively evaluated for impairment	3,119		3,003
Purchased credit impaired loans	0		0
Total allowance on loan losses	4,244	4,190	3,864	3,686	3,421	3,318
Loans individually evaluated for impairment	8,037		8,874
Loans collectively evaluated for impairment	630,587		595,007
Purchased credit impaired loans	5,578		5,756
Total Loans	644,202		609,637
Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans individually evaluated for impairment			92
Loans collectively evaluated for impairment	944		786
Purchased credit impaired loans	0		0
Total allowance on loan losses	944	1,043	878	456	528	493
Loans individually evaluated for impairment			92
Loans collectively evaluated for impairment	124,563		114,001
Total Loans	124,563		114,093
Consumer and Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans individually evaluated for impairment	131		141
Loans collectively evaluated for impairment	1,794		2,887
Purchased credit impaired loans	0		0
Total allowance on loan losses	1,925	$ 2,690	3,028	$ 99	$ 44	$ 52
Loans individually evaluated for impairment	131		141
Loans collectively evaluated for impairment	32,576		42,356
Purchased credit impaired loans	60		69
Total Loans	$ 32,767		$ 42,566