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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	$ 29,524	$ 28,143	$ 30,323	$ 32,503	$ 32,503
Charge-offs	(1,142)	(861)	(1,940)	(3,782)	(4,410)
Recoveries	570	700	1,443	1,614	2,141
Provision (benefit)	2,651	765	1,777	(1,588)	89
Ending Balance	31,603	28,747	31,603	28,747	30,323
Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	13,598	12,614	13,758	14,265	14,265
Charge-offs	(25)	(80)	(92)	(230)	(246)
Recoveries	15	238	51	365	397
Provision (benefit)	1,691	816	1,562	(812)	(658)
Ending Balance	15,279	13,588	15,279	13,588	13,758
Mortgage Loans on Real Estate [Member] | Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	11,607	10,119	11,441	11,517	11,517
Charge-offs		(20)	(15)	(170)	(186)
Recoveries	15	238	51	365	397
Provision (benefit)	1,257	1,033	1,402	(342)	(287)
Ending Balance	12,879	11,370	12,879	11,370	11,441
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	7,137	9,155	8,227	10,310	10,310
Charge-offs	(424)	(457)	(896)	(1,260)	(1,616)
Recoveries	353	401	1,061	933	1,315
Provision (benefit)	448	(797)	(878)	(1,681)	(1,782)
Ending Balance	7,514	8,302	7,514	8,302	8,227
Consumer [Member] | Home Equity Lines of Credit [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	5,048	6,156	5,800	7,044	7,044
Charge-offs	(172)	(14)	(276)	(98)	(98)
Recoveries	151	189	677	487	698
Provision (benefit)	194	(610)	(980)	(1,712)	(1,844)
Ending Balance	5,221	5,721	5,221	5,721	5,800
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	6,378	4,729	6,512	5,831	5,831
Charge-offs	(693)	(291)	(952)	(1,188)	(1,444)
Recoveries	202	61	331	315	428
Provision (benefit)	337	303	333	(156)	1,697
Ending Balance	6,224	4,802	6,224	4,802	6,512
Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	2,411	1,645	1,826	2,097	2,097
Charge-offs		(33)		(1,104)	(1,104)
Recoveries				1	1
Provision (benefit)	175	443	760	1,061	832
Ending Balance	2,586	2,055	2,586	2,055	1,826
Construction [Member] | Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	977	466	642	680	680
Recoveries				1	1
Provision (benefit)	(17)	159	318	(56)	(39)
Ending Balance	960	625	960	625	642
Construction [Member] | Residential [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	1,434	1,179	1,184	1,417	1,417
Charge-offs		(33)		(1,104)	(1,104)
Provision (benefit)	192	284	442	1,117	871
Ending Balance	1,626	1,430	1,626	1,430	1,184
Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	1,991	2,495	2,317	2,748	2,748
Charge-offs	(25)	(60)	(77)	(60)	(60)
Provision (benefit)	434	(217)	160	(470)	(371)
Ending Balance	2,400	2,218	2,400	2,218	2,317
Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	1,532	2,354	1,841	2,644	2,644
Charge-offs	(23)	(94)	(23)	(331)	(332)
Recoveries	139	121	176	146	242
Provision (benefit)	(55)	(390)	(401)	(468)	(713)
Ending Balance	1,593	1,991	1,593	1,991	1,841
Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	557	645	586	622	622
Charge-offs	(229)	(349)	(597)	(831)	(1,186)
Recoveries	63	91	208	300	375
Provision (benefit)	309	203	503	499	775
Ending Balance	$ 700	$ 590	$ 700	$ 590	$ 586