Allowance for Loan Losses - Summary of Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	$ 28,848	$ 26,695	$ 28,543	$ 26,255
Charge-offs	(75)	(72)	(218)	(159)
Recoveries	47	71	120	148
Provision	250	450	625	900
Allowance for loan losses, end of year	29,070	27,144	29,070	27,144
Amount of allowance for loan losses for loans deemed to be impaired	333	713	333	713
Amount of allowance for loan losses for loans not deemed to be impaired	28,737	26,431	28,737	26,431
Loans:
Total loans, net	2,312,571	2,221,613	2,312,571	2,221,613	$ 2,285,578
Loans deemed to be impaired	3,913	5,772	3,913	5,772
Loans not deemed to be impaired	2,308,658	2,215,841	2,308,658	2,215,841
Construction and Land Development [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	1,011	1,438	1,092	1,645
Provision	41	(805)	(40)	(1,012)
Allowance for loan losses, end of year	1,052	633	1,052	633
Amount of allowance for loan losses for loans not deemed to be impaired	1,052	633	1,052	633
Loans:
Total loans, net	13,751	7,729	13,751	7,729	13,628
Loans not deemed to be impaired	13,751	7,729	13,751	7,729
Commercial and Industrial [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	11,156	9,664	10,998	9,651
Charge-offs	(8)		(51)	(5)
Recoveries	7	10	25	33
Provision	183	710	366	705
Allowance for loan losses, end of year	11,338	10,384	11,338	10,384
Amount of allowance for loan losses for loans deemed to be impaired	248	47	248	47
Amount of allowance for loan losses for loans not deemed to be impaired	11,090	10,337	11,090	10,337
Loans:
Total loans, net	764,492	761,467	764,492	761,467	761,625
Loans deemed to be impaired	548	592	548	592
Loans not deemed to be impaired	763,944	760,875	763,944	760,875
Municipal [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	1,998	1,720	1,838	1,720
Provision	(166)	(16)	(6)	(16)
Allowance for loan losses, end of year	1,832	1,704	1,832	1,704
Amount of allowance for loan losses for loans not deemed to be impaired	1,832	1,704	1,832	1,704
Loans:
Total loans, net	95,682	103,027	95,682	103,027	97,290
Loans not deemed to be impaired	95,682	103,027	95,682	103,027
Commercial Real Estate [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	10,767	9,787	10,663	9,728
Provision	81	422	185	481
Allowance for loan losses, end of year	10,848	10,209	10,848	10,209
Amount of allowance for loan losses for loans deemed to be impaired	85	91	85	91
Amount of allowance for loan losses for loans not deemed to be impaired	10,763	10,118	10,763	10,118
Loans:
Total loans, net	758,242	735,083	758,242	735,083	750,362
Loans deemed to be impaired	3,365	2,505	3,365	2,505
Loans not deemed to be impaired	754,877	732,578	754,877	732,578
Residential Real Estate [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	2,135	2,459	2,190	1,873
Provision	75	34	20	620
Allowance for loan losses, end of year	2,210	2,493	2,210	2,493
Amount of allowance for loan losses for loans deemed to be impaired	575	575	575	575
Amount of allowance for loan losses for loans not deemed to be impaired	2,210	1,918	2,210	1,918
Loans:
Total loans, net	346,585	316,248	346,585	316,248	348,250
Loans deemed to be impaired		2,675		2,675
Loans not deemed to be impaired	346,585	313,573	346,585	313,573
Consumer [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	342	321	365	373
Charge-offs	(67)	(72)	(167)	(154)
Recoveries	40	61	95	115
Provision	65	26	87	2
Allowance for loan losses, end of year	380	336	380	336
Amount of allowance for loan losses for loans not deemed to be impaired	380	336	380	336
Loans:
Total loans, net	22,609	21,662	22,609	21,662	22,083
Loans not deemed to be impaired	22,609	21,662	22,609	21,662
Home Equity [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	1,101	1,067	1,111	989
Provision	19	11	9	89
Allowance for loan losses, end of year	1,120	1,078	1,120	1,078
Amount of allowance for loan losses for loans not deemed to be impaired	1,120	1,078	1,120	1,078
Loans:
Total loans, net	311,210	276,397	311,210	276,397	$ 292,340
Loans not deemed to be impaired	311,210	276,397	311,210	276,397
Unallocated [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	338	239	286	276
Provision	(48)	68	4	31
Allowance for loan losses, end of year	290	307	290	307
Amount of allowance for loan losses for loans not deemed to be impaired	$ 290	$ 307	$ 290	$ 307