Allowance for Loan Losses - Summary of Activity in Allowance for Loan Losses, and Ending Balance of Loans, Net of Unearned Fees for Periods Indicated (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	$ 29,973	$ 31,017	$ 30,323	$ 32,503	$ 32,503
Charge-offs	(318)	(2,512)	(798)	(2,921)	(4,410)
Recoveries	507	434	873	914	2,141
Provision (benefit)	(638)	(796)	(874)	(2,353)	89
Ending Balance	29,524	28,143	29,524	28,143	30,323
Allowance for Loan Losses, Individually evaluated for impairment						$ 2,889	$ 2,699	$ 1,686
Allowance for Loan Losses, Loans pooled for evaluation						26,496	27,352	24,026
Total allowance for loan losses	29,973	31,017	29,524	28,143	32,503	29,524	30,323	28,143
Loans, Net of Unearned fees , Individually evaluated for impairment						27,211	28,234	26,235
Loans, Net of Unearned fees , Loans pooled for evaluation						3,105,241	2,971,371	2,779,866
Total loans, net of unamortized deferred loan fees and discounts						3,146,313	3,015,165	2,826,393
Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	13,665	14,204	13,758	14,265	14,265
Charge-offs	(66)	(150)	(67)	(150)	(246)
Recoveries	21	17	36	127	397
Provision (benefit)	(22)	(1,458)	(129)	(1,629)	(658)
Ending Balance	13,598	12,613	13,598	12,613	13,758
Allowance for Loan Losses, Individually evaluated for impairment						229	260	373
Allowance for Loan Losses, Loans pooled for evaluation						13,260	13,283	10,568
Total allowance for loan losses	13,665	14,204	13,598	12,613	14,265	13,598	13,758	12,613
Loans, Net of Unearned fees , Individually evaluated for impairment						17,506	19,209	19,250
Loans, Net of Unearned fees , Loans pooled for evaluation						2,374,219	2,271,165	2,075,263
Total loans, net of unamortized deferred loan fees and discounts						2,401,040	2,300,322	2,106,567
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	7,718	9,576	8,227	10,310	10,310
Charge-offs	(198)	(527)	(472)	(803)	(1,616)
Recoveries	406	333	707	532	1,315
Provision (benefit)	(789)	(225)	(1,325)	(882)	(1,782)
Ending Balance	7,137	9,157	7,137	9,157	8,227
Allowance for Loan Losses, Individually evaluated for impairment						533	591	488
Allowance for Loan Losses, Loans pooled for evaluation						6,597	7,619	8,595
Total allowance for loan losses	7,718	9,576	7,137	9,157	10,310	7,137	8,227	9,157
Loans, Net of Unearned fees , Individually evaluated for impairment						4,954	4,415	4,241
Loans, Net of Unearned fees , Loans pooled for evaluation						343,898	349,431	352,168
Total loans, net of unamortized deferred loan fees and discounts						350,925	356,874	360,782
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	6,392	5,326	6,512	5,831	5,831
Charge-offs	(54)	(764)	(259)	(897)	(1,444)
Recoveries	80	84	130	254	428
Provision (benefit)	(40)	83	(5)	(459)	1,697
Ending Balance	6,378	4,729	6,378	4,729	6,512
Allowance for Loan Losses, Individually evaluated for impairment						2,127	1,848	811
Allowance for Loan Losses, Loans pooled for evaluation						4,228	4,624	3,276
Total allowance for loan losses	6,392	5,326	6,378	4,729	5,831	6,378	6,512	4,729
Loans, Net of Unearned fees , Individually evaluated for impairment						4,751	4,470	2,744
Loans, Net of Unearned fees , Loans pooled for evaluation						230,395	213,358	219,658
Total loans, net of unamortized deferred loan fees and discounts						237,619	220,412	225,743
Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	2,198	1,911	1,826	2,097	2,097
Charge-offs		(1,071)		(1,071)	(1,104)
Recoveries				1	1
Provision (benefit)	213	804	585	617	832
Ending Balance	2,411	1,644	2,411	1,644	1,826
Allowance for Loan Losses, Individually evaluated for impairment								14
Allowance for Loan Losses, Loans pooled for evaluation						2,411	1,826	1,587
Total allowance for loan losses	2,198	1,911	2,411	1,644	2,097	2,411	1,826	1,644
Loans, Net of Unearned fees , Individually evaluated for impairment							140
Loans, Net of Unearned fees , Loans pooled for evaluation						156,729	137,417	132,777
Total loans, net of unamortized deferred loan fees and discounts						156,729	137,557	133,301
Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	2,170	2,662	2,317	2,748	2,748
Charge-offs	(51)		(52)		(60)
Provision (benefit)	(128)	(151)	(274)	(237)	(371)
Ending Balance	1,991	2,511	1,991	2,511	2,317
Allowance for Loan Losses, Individually evaluated for impairment						147	230	249
Allowance for Loan Losses, Loans pooled for evaluation						1,794	1,932	2,037
Total allowance for loan losses	2,170	2,662	1,991	2,511	2,748	1,991	2,317	2,511
Loans, Net of Unearned fees , Individually evaluated for impairment						6,344	5,298	4,726
Loans, Net of Unearned fees , Loans pooled for evaluation						376,628	378,743	375,217
Total loans, net of unamortized deferred loan fees and discounts						384,692	385,426	381,305
Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	1,736	2,451	1,841	2,644	2,644
Charge-offs		(206)		(237)	(332)
Recoveries	23	13	37	25	242
Provision (benefit)	(227)	98	(346)	(76)	(713)
Ending Balance	1,532	2,356	1,532	2,356	1,841
Allowance for Loan Losses, Individually evaluated for impairment						192	107	57
Allowance for Loan Losses, Loans pooled for evaluation						1,340	1,734	2,233
Total allowance for loan losses	1,736	2,451	1,532	2,356	2,644	1,532	1,841	2,356
Loans, Net of Unearned fees , Individually evaluated for impairment						2,457	1,470	1,285
Loans, Net of Unearned fees , Loans pooled for evaluation						38,074	41,076	43,515
Total loans, net of unamortized deferred loan fees and discounts						40,986	43,031	45,936
Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	570	595	586	622	622
Charge-offs	(174)	(308)	(368)	(482)	(1,186)
Recoveries	66	68	144	209	375
Provision (benefit)	95	290	195	296	775
Ending Balance	557	645	557	645	586
Allowance for Loan Losses, Individually evaluated for impairment						54	57	31
Allowance for Loan Losses, Loans pooled for evaluation						503	529	614
Total allowance for loan losses	570	595	557	645	622	557	586	645
Loans, Net of Unearned fees , Individually evaluated for impairment						247	257	323
Loans, Net of Unearned fees , Loans pooled for evaluation						23,213	24,853	27,981
Total loans, net of unamortized deferred loan fees and discounts						23,503	25,155	28,370
Commercial [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	11,495	11,542	11,441	11,517	11,517
Charge-offs	(15)	(150)	(15)	(150)	(186)
Recoveries	21	17	36	127	397
Provision (benefit)	106	(1,307)	145	(1,392)	(287)
Ending Balance	11,607	10,102	11,607	10,102	11,441
Allowance for Loan Losses, Individually evaluated for impairment						82	30	124
Allowance for Loan Losses, Loans pooled for evaluation						11,466	11,351	8,531
Total allowance for loan losses	11,495	11,542	11,607	10,102	11,517	11,607	11,441	10,102
Loans, Net of Unearned fees , Individually evaluated for impairment						11,162	13,911	14,524
Loans, Net of Unearned fees , Loans pooled for evaluation						1,997,591	1,892,422	1,700,046
Total loans, net of unamortized deferred loan fees and discounts						2,016,348	1,914,896	1,725,262
Commercial [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	847	572	642	680	680
Recoveries				1	1
Provision (benefit)	130	(106)	335	(215)	(39)
Ending Balance	977	466	977	466	642
Allowance for Loan Losses, Loans pooled for evaluation						977	642	466
Total allowance for loan losses	847	572	977	466	680	977	642	466
Loans, Net of Unearned fees , Loans pooled for evaluation						83,151	69,627	70,151
Total loans, net of unamortized deferred loan fees and discounts						83,151	69,627	70,151
Home Equity Lines of Credit [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	5,412	6,530	5,800	7,044	7,044
Charge-offs	(24)	(13)	(104)	(84)	(98)
Recoveries	317	252	526	298	698
Provision (benefit)	(657)	(613)	(1,174)	(1,102)	(1,844)
Ending Balance	5,048	6,156	5,048	6,156	5,800
Allowance for Loan Losses, Individually evaluated for impairment						287	427	400
Allowance for Loan Losses, Loans pooled for evaluation						4,754	5,356	5,748
Total allowance for loan losses	5,412	6,530	5,048	6,156	7,044	5,048	5,800	6,156
Loans, Net of Unearned fees , Individually evaluated for impairment						2,250	2,688	2,633
Loans, Net of Unearned fees , Loans pooled for evaluation						282,611	283,502	280,672
Total loans, net of unamortized deferred loan fees and discounts						286,436	288,688	286,476
Residential [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning Balance	1,351	1,339	1,184	1,417	1,417
Charge-offs		(1,071)		(1,071)	(1,104)
Provision (benefit)	83	910	250	832	871
Ending Balance	1,434	1,178	1,434	1,178	1,184
Allowance for Loan Losses, Individually evaluated for impairment								14
Allowance for Loan Losses, Loans pooled for evaluation						1,434	1,184	1,121
Total allowance for loan losses	$ 1,351	$ 1,339	$ 1,434	$ 1,178	$ 1,417	1,434	1,184	1,178
Loans, Net of Unearned fees , Individually evaluated for impairment							140
Loans, Net of Unearned fees , Loans pooled for evaluation						73,578	67,790	62,626
Total loans, net of unamortized deferred loan fees and discounts						73,578	67,930	63,150
Financial Asset Acquired with Credit Deterioration [Member]
Loans and Leases Receivable Disclosure [Line Items]
Allowance for Loan Losses, Loans acquired with deteriorated credit quality						139	272	2,431
Total loans, net of unamortized deferred loan fees and discounts						13,861	15,560	20,292
Financial Asset Acquired with Credit Deterioration [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Allowance for Loan Losses, Loans acquired with deteriorated credit quality						109	215	1,672
Total loans, net of unamortized deferred loan fees and discounts						9,315	9,948	12,054
Financial Asset Acquired with Credit Deterioration [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Allowance for Loan Losses, Loans acquired with deteriorated credit quality						7	17	74
Total loans, net of unamortized deferred loan fees and discounts						2,073	3,028	4,373
Financial Asset Acquired with Credit Deterioration [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Allowance for Loan Losses, Loans acquired with deteriorated credit quality						23	40	642
Total loans, net of unamortized deferred loan fees and discounts						2,473	2,584	3,341
Financial Asset Acquired with Credit Deterioration [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Allowance for Loan Losses, Loans acquired with deteriorated credit quality								43
Total loans, net of unamortized deferred loan fees and discounts								524
Financial Asset Acquired with Credit Deterioration [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Allowance for Loan Losses, Loans acquired with deteriorated credit quality						50	155	225
Total loans, net of unamortized deferred loan fees and discounts						1,720	1,385	1,362
Financial Asset Acquired with Credit Deterioration [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Allowance for Loan Losses, Loans acquired with deteriorated credit quality								66
Total loans, net of unamortized deferred loan fees and discounts						455	485	1,136
Financial Asset Acquired with Credit Deterioration [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts						43	45	66
Financial Asset Acquired with Credit Deterioration [Member] | Commercial [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Allowance for Loan Losses, Loans acquired with deteriorated credit quality						59	60	1,447
Total loans, net of unamortized deferred loan fees and discounts						7,595	8,563	10,692
Financial Asset Acquired with Credit Deterioration [Member] | Home Equity Lines of Credit [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Allowance for Loan Losses, Loans acquired with deteriorated credit quality						7	17	8
Total loans, net of unamortized deferred loan fees and discounts						$ 1,575	$ 2,498	3,171
Financial Asset Acquired with Credit Deterioration [Member] | Residential [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Allowance for Loan Losses, Loans acquired with deteriorated credit quality								43
Total loans, net of unamortized deferred loan fees and discounts								$ 524