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		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 2,931,613	$ 2,712,226	$ 2,931,613	$ 2,712,226	$ 2,759,593
Individ. evaluated for impairment	32,225	29,276	32,225	29,276	28,140
Loans pooled for evaluation	2,883,384	2,656,067	2,883,384	2,656,067	2,706,409
Beginning balance	28,143	35,509	32,503	36,011	36,011
Charge-offs	(861)	(664)	(3,782)	(2,594)	(3,230)
Recoveries	700	2,612	1,614	4,604	5,692
(Benefit) provision	765	(3,973)	(1,588)	(4,537)	(5,970)
Ending balance	28,747	33,484	28,747	33,484	32,503
Ending balance:
Individ. evaluated for impairment	2,140	2,778	2,140	2,778	2,046
Loans pooled for evaluation	26,320	27,946	26,320	27,946	27,760
Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,194,874		2,194,874		2,053,464
Mortgage Loans on Real Estate [Member] | Residential [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	391,072	368,223	391,072	368,223	368,343
Individ. evaluated for impairment	5,027	5,414	5,027	5,414	4,094
Loans pooled for evaluation	384,640	361,336	384,640	361,336	362,780
Beginning balance	2,495	3,081	2,761	2,896	2,896
Charge-offs	(60)	(50)	(60)	(212)	(321)
Recoveries		391		618	880
(Benefit) provision	(217)	(653)	(470)	(533)	(694)
Ending balance	2,218	2,769	2,218	2,769	2,761
Ending balance:
Individ. evaluated for impairment	240	418	240	418	258
Loans pooled for evaluation	1,978	2,157	1,978	2,157	2,317
Mortgage Loans on Real Estate [Member] | Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,803,802	1,625,897	1,803,802	1,625,897	1,685,121
Individ. evaluated for impairment	19,788	14,706	19,788	14,706	15,081
Loans pooled for evaluation	1,775,843	1,597,876	1,775,843	1,597,876	1,657,238
Beginning balance	10,119	11,934	11,503	11,015	11,015
Charge-offs	(20)		(170)	(793)	(827)
Recoveries	238	20	365	902	920
(Benefit) provision	1,033	(378)	(342)	452	395
Ending balance	11,370	11,576	11,370	11,576	11,503
Ending balance:
Individ. evaluated for impairment	73	644	73	644	4
Loans pooled for evaluation	11,022	9,493	11,022	9,493	10,050
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	361,320		361,320		366,663
Consumer [Member] | Home Equity Lines of Credit [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	289,506	301,884	289,506	301,884	289,715
Individ. evaluated for impairment	2,219	3,820	2,219	3,820	3,196
Loans pooled for evaluation	284,335	292,833	284,335	292,833	282,159
Beginning balance	6,156	9,203	7,045	11,253	11,253
Charge-offs	(14)	(122)	(98)	(450)	(585)
Recoveries	189	1,580	487	1,921	2,317
(Benefit) provision	(610)	(2,261)	(1,712)	(4,324)	(5,940)
Ending balance	5,721	8,400	5,721	8,400	7,045
Ending balance:
Individ. evaluated for impairment	363	511	363	511	411
Loans pooled for evaluation	5,347	7,864	5,347	7,864	6,617
Consumer and Industrial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	227,479	217,110	227,479	217,110	217,047
Individ. evaluated for impairment	2,938	2,397	2,938	2,397	4,096
Loans pooled for evaluation	221,846	210,565	221,846	210,565	208,960
Beginning balance	4,729	5,265	5,831	5,271	5,271
Charge-offs	(291)	(307)	(1,188)	(421)	(455)
Recoveries	61	85	315	323	404
(Benefit) provision	303	199	(156)	69	611
Ending balance	4,802	5,242	4,802	5,242	5,831
Ending balance:
Individ. evaluated for impairment	1,276	822	1,276	822	1,130
Loans pooled for evaluation	3,526	3,426	3,526	3,426	3,765
Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	147,940		147,940		122,419
Construction [Member] | Residential [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	75,120	57,892	75,120	57,892	55,429
Individ. evaluated for impairment	144	11	144	11	11
Loans pooled for evaluation	74,976	57,338	74,976	57,338	54,743
Beginning balance	1,179	1,321	1,416	899	899
Charge-offs	(33)		(1,104)
Recoveries					54
(Benefit) provision	284	(16)	1,117	406	463
Ending balance	1,430	1,305	1,430	1,305	1,416
Ending balance:
Loans pooled for evaluation	1,430	1,257	1,430	1,257	1,371
Construction [Member] | Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	72,820	61,295	72,820	61,295	66,990
Loans pooled for evaluation	72,820	61,295	72,820	61,295	66,990
Beginning balance	466	952	680	812	812
Recoveries			1	1	78
(Benefit) provision	159	(528)	(56)	(389)	(210)
Ending balance	625	424	625	424	680
Ending balance:
Loans pooled for evaluation	625	424	625	424	680
Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	45,033	48,314	45,033	48,314	46,182
Individ. evaluated for impairment	1,842	2,650	1,842	2,650	1,508
Loans pooled for evaluation	42,454	43,552	42,454	43,552	42,992
Beginning balance	2,354	3,057	2,645	3,177	3,177
Charge-offs	(94)	(25)	(331)	(118)	(219)
Recoveries	121	429	146	501	590
(Benefit) provision	(390)	(360)	(468)	(459)	(903)
Ending balance	1,991	3,101	1,991	3,101	2,645
Ending balance:
Individ. evaluated for impairment	111	311	111	311	215
Loans pooled for evaluation	1,879	2,730	1,879	2,730	2,366
Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	26,781	31,611	26,781	31,611	30,766
Individ. evaluated for impairment	267	278	267	278	154
Loans pooled for evaluation	26,470	31,272	26,470	31,272	30,547
Beginning balance	645	696	622	688	688
Charge-offs	(349)	(160)	(831)	(600)	(823)
Recoveries	91	107	300	338	449
(Benefit) provision	203	24	499	241	308
Ending balance	590	667	590	667	622
Ending balance:
Individ. evaluated for impairment	77	72	77	72	28
Loans pooled for evaluation	513	595	513	595	594
Receivables Acquired with Deteriorated Credit Quality [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	16,004	26,883	16,004	26,883	25,044
Ending balance:
Loans acquired with deteriorated credit quality	287	2,760	287	2,760	2,697
Receivables Acquired with Deteriorated Credit Quality [Member] | Mortgage Loans on Real Estate [Member] | Residential [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,405	1,473	1,405	1,473	1,469
Ending balance:
Loans acquired with deteriorated credit quality		181		181	186
Receivables Acquired with Deteriorated Credit Quality [Member] | Mortgage Loans on Real Estate [Member] | Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	8,171	13,315	8,171	13,315	12,802
Ending balance:
Loans acquired with deteriorated credit quality	275	1,453	275	1,453	1,449
Receivables Acquired with Deteriorated Credit Quality [Member] | Consumer [Member] | Home Equity Lines of Credit [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,952	5,231	2,952	5,231	4,360
Ending balance:
Loans acquired with deteriorated credit quality	12	25	12	25	17
Receivables Acquired with Deteriorated Credit Quality [Member] | Consumer and Industrial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,695	4,148	2,695	4,148	3,991
Ending balance:
Loans acquired with deteriorated credit quality		994		994	936
Receivables Acquired with Deteriorated Credit Quality [Member] | Construction [Member] | Residential [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts		543		543	675
Ending balance:
Loans acquired with deteriorated credit quality		48		48	45
Receivables Acquired with Deteriorated Credit Quality [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	737	2,112	737	2,112	1,682
Ending balance:
Loans acquired with deteriorated credit quality		59		59	64
Receivables Acquired with Deteriorated Credit Quality [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 44	$ 61	$ 44	$ 61	$ 65