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Allowance for Loan Losses - Summary of Activity in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation						$ 30,007	$ 30,266	$ 28,865
Beginning Balance	$ 32,868	$ 29,524	$ 32,582	$ 30,323	$ 30,323
Charge-offs	(1,522)	(1,142)	(2,541)	(1,940)	(2,364)
Recoveries	520	570	2,888	1,443	2,040
Provision (benefit)	(329)	2,651	(1,392)	1,777	2,583
Ending Balance	31,537	31,603	31,537	31,603	32,582
Individually evaluated for impairment						1,524	2,194	2,628
Total allowance for loan losses	32,868	29,524	31,537	30,323	32,582	31,537	32,582	31,603
Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation						14,230	15,357	14,865
Beginning Balance	14,675	13,598	15,620	13,758	13,758
Charge-offs	(746)	(25)	(748)	(92)	(92)
Recoveries	174	15	1,570	51	68
Provision (benefit)	244	1,691	(2,095)	1,562	1,886
Ending Balance	14,347	15,279	14,347	15,279	15,620
Individually evaluated for impairment						117	147	325
Total allowance for loan losses	14,675	13,598	15,620	13,758	15,620	14,347	15,620	15,279
Mortgage Loans on Real Estate \| Commercial loans
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation						11,915	12,737	12,552
Beginning Balance	12,099	11,607	12,944	11,441	11,441
Charge-offs	(746)	0	(746)	(15)	(15)
Recoveries	126	15	1,517	51	68
Provision (benefit)	462	1,257	(1,774)	1,402	1,450
Ending Balance	11,941	12,879	11,941	12,879	12,944
Individually evaluated for impairment						26	91	268
Total allowance for loan losses	12,099	11,607	11,941	11,441	11,441	11,941	12,944	12,879
Consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation						8,395	8,103	7,061
Beginning Balance	8,552	7,137	8,375	8,227	8,227
Charge-offs	(191)	(424)	(551)	(896)	(1,084)
Recoveries	262	353	981	1,061	1,431
Provision (benefit)	(74)	448	(256)	(878)	(199)
Ending Balance	8,549	7,514	8,549	7,514	8,375
Individually evaluated for impairment						148	266	446
Total allowance for loan losses	8,552	7,137	8,375	7,514	8,375	8,549	8,375	7,514
Consumer \| Home equity lines of credit
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation						5,644	5,838	5,046
Beginning Balance	5,859	5,048	6,042	5,800	5,800
Charge-offs		(172)		(276)	(277)
Recoveries	27	151	305	677	846
Provision (benefit)	(152)	194	(613)	(980)	(327)
Ending Balance	5,734	5,221	5,734	5,221	6,042
Individually evaluated for impairment						84	198	168
Total allowance for loan losses	5,859	5,048	5,734	5,800	5,800	5,734	6,042	5,221
Commercial
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation						4,428	4,309	4,353
Beginning Balance	6,745	6,378	6,090	6,512	6,512
Charge-offs	(585)	(693)	(1,242)	(952)	(1,188)
Recoveries	84	202	337	331	541
Provision (benefit)	(557)	337	502	333	225
Ending Balance	5,687	6,224	5,687	6,224	6,090
Individually evaluated for impairment						1,259	1,781	1,857
Total allowance for loan losses	6,745	6,378	6,090	6,512	6,090	5,687	6,090	6,224
Construction
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation						2,954	2,497	2,586
Beginning Balance	2,896	2,411	2,497	1,826	1,826
Provision (benefit)	58	175	457	760	671
Ending Balance	2,954	2,586	2,954	2,586	2,497
Total allowance for loan losses	2,896	2,411	2,497	1,826	2,497	2,954	2,497	2,586
Construction \| Commercial loans
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation						297	663	960
Beginning Balance	358	977	663	642	642
Provision (benefit)	(61)	(17)	(366)	318	21
Ending Balance	297	960	297	960	663
Total allowance for loan losses	358	977	297	642	663	297	663	960
Construction \| Residential
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation						2,657	1,834	1,626
Beginning Balance	2,538	1,434	1,834	1,184	1,184
Provision (benefit)	119	192	823	442	650
Ending Balance	2,657	1,626	2,657	1,626	1,834
Total allowance for loan losses	2,538	1,434	2,657	1,184	1,834	2,657	1,834	1,626
Residential 1-4 family \| Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation						2,315	2,620	2,313
Beginning Balance	2,576	1,991	2,676	2,317	2,317
Charge-offs		(25)	(2)	(77)	(77)
Recoveries	48		53
Provision (benefit)	(218)	434	(321)	160	436
Ending Balance	2,406	2,400	2,406	2,400	2,676
Individually evaluated for impairment						91	56	57
Total allowance for loan losses	2,576	1,991	2,676	2,400	2,676	2,406	2,676	2,400
Home equity loans \| Consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation						1,216	1,486	1,418
Beginning Balance	1,242	1,532	1,540	1,841	1,841
Charge-offs	(3)	(23)	(3)	(23)	(24)
Recoveries	156	139	414	176	297
Provision (benefit)	(133)	(55)	(689)	(401)	(574)
Ending Balance	1,262	1,593	1,262	1,593	1,540
Individually evaluated for impairment						46	54	175
Total allowance for loan losses	1,242	1,532	1,262	1,841	1,540	1,262	1,540	1,593
Other \| Consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation						1,535	779	597
Beginning Balance	1,451	557	793	586	586
Charge-offs	(188)	(229)	(548)	(597)	(783)
Recoveries	79	63	262	208	288
Provision (benefit)	211	309	1,046	503	702
Ending Balance	1,553	700	1,553	700	793
Individually evaluated for impairment						18	14	103
Total allowance for loan losses	$ 1,451	$ 557	$ 1,553	$ 586	$ 793	1,553	793	700
Loans acquired with deteriorated credit quality
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality						6	122	110
Loans acquired with deteriorated credit quality \| Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality							116	89
Loans acquired with deteriorated credit quality \| Mortgage Loans on Real Estate \| Commercial loans
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality							116	59
Loans acquired with deteriorated credit quality \| Consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality						6	6	7
Loans acquired with deteriorated credit quality \| Consumer \| Home equity lines of credit
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality						$ 6	$ 6	7
Loans acquired with deteriorated credit quality \| Commercial
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality								14
Loans acquired with deteriorated credit quality \| Residential 1-4 family \| Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality								$ 30