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	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 3,015,165	$ 2,759,593	$ 2,522,937
Individ. evaluated for impairment	28,234	28,140	50,105
Loans pooled for evaluation	2,971,371	2,706,409	2,432,628
Beginning balance	32,503	36,011	36,585
Charge-offs	(4,410)	(3,230)	(2,816)
Recoveries	2,141	5,692	4,452
(Benefit) provision	89	(5,970)	(2,210)
Ending balance	30,323	32,503	36,011
Ending balance:
Individ. evaluated for impairment	2,699	2,046	2,891
Loans pooled for evaluation	27,352	27,760	30,227
Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,300,322	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	385,426	368,343	314,265
Individ. evaluated for impairment	5,298	4,094	6,767
Loans pooled for evaluation	378,743	362,780	305,353
Beginning balance	2,748	2,896	3,086
Charge-offs	(60)	(321)	(224)
Recoveries		880	204
(Benefit) provision	(371)	(707)	(170)
Ending balance	2,317	2,748	2,896
Ending balance:
Individ. evaluated for impairment	230	258	335
Loans pooled for evaluation	1,932	2,304	2,501
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	1,914,896	1,685,121	1,497,567
Individ. evaluated for impairment	13,911	15,081	32,407
Loans pooled for evaluation	1,892,422	1,657,238	1,442,100
Beginning balance	11,517	11,015	9,227
Charge-offs	(186)	(827)
Recoveries	397	920	243
(Benefit) provision	(287)	409	1,545
Ending balance	11,441	11,517	11,015
Ending balance:
Individ. evaluated for impairment	30	4	396
Loans pooled for evaluation	11,351	10,064	9,167
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	356,874	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	288,688	289,715	322,492
Individ. evaluated for impairment	2,688	3,196	5,747
Loans pooled for evaluation	283,502	282,159	309,007
Beginning balance	7,044	11,253	15,676
Charge-offs	(98)	(585)	(694)
Recoveries	698	2,317	666
(Benefit) provision	(1,844)	(5,941)	(4,395)
Ending balance	5,800	7,044	11,253
Ending balance:
Individ. evaluated for impairment	427	411	605
Loans pooled for evaluation	5,356	6,616	10,423
Consumer and Industrial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	220,412	217,047	194,913
Individ. evaluated for impairment	4,470	4,096	2,671
Loans pooled for evaluation	213,358	208,960	187,393
Beginning balance	5,831	5,271	4,226
Charge-offs	(1,444)	(455)	(680)
Recoveries	428	404	677
(Benefit) provision	1,697	611	1,048
Ending balance	6,512	5,831	5,271
Ending balance:
Individ. evaluated for impairment	1,848	1,130	1,187
Loans pooled for evaluation	4,624	3,765	2,983
Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	137,557	122,419
Construction [Member] | Residential [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	67,930	55,429	46,135
Individ. evaluated for impairment	140	11	4
Loans pooled for evaluation	67,790	54,743	45,410
Beginning balance	1,417	899	1,434
Charge-offs	(1,104)
Recoveries		54	1,728
(Benefit) provision	871	464	(2,263)
Ending balance	1,184	1,417	899
Ending balance:
Loans pooled for evaluation	1,184	1,372	844
Construction [Member] | Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	69,627	66,990	74,774
Individ. evaluated for impairment			490
Loans pooled for evaluation	69,627	66,990	74,284
Beginning balance	680	812	411
Recoveries	1	78	140
(Benefit) provision	(39)	(210)	261
Ending balance	642	680	812
Ending balance:
Loans pooled for evaluation	642	680	812
Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	43,031	46,182	40,362
Individ. evaluated for impairment	1,470	1,508	1,731
Loans pooled for evaluation	41,076	42,992	37,004
Beginning balance	2,644	3,177	1,797
Charge-offs	(332)	(219)	(242)
Recoveries	242	590	252
(Benefit) provision	(713)	(904)	1,370
Ending balance	1,841	2,644	3,177
Ending balance:
Individ. evaluated for impairment	107	215	294
Loans pooled for evaluation	1,734	2,365	2,883
Auto Indirect [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning balance			9
Charge-offs			(4)
Recoveries			42
(Benefit) provision			(47)
Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	25,155	30,766	32,429
Individ. evaluated for impairment	257	154	288
Loans pooled for evaluation	24,853	30,547	32,077
Beginning balance	622	688	719
Charge-offs	(1,186)	(823)	(972)
Recoveries	375	449	500
(Benefit) provision	775	308	441
Ending balance	586	622	688
Ending balance:
Individ. evaluated for impairment	57	28	74
Loans pooled for evaluation	529	594	614
Receivables Acquired with Deteriorated Credit Quality [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	15,560	25,044	40,204
Ending balance:
Loans acquired with deteriorated credit quality	272	2,697	2,893
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	1,385	1,469	2,145
Ending balance:
Loans acquired with deteriorated credit quality	155	186	60
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	8,563	12,802	23,060
Ending balance:
Loans acquired with deteriorated credit quality	60	1,449	1,452
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	2,498	4,360	7,738
Ending balance:
Loans acquired with deteriorated credit quality	17	17	225
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	2,584	3,991	4,849
Ending balance:
Loans acquired with deteriorated credit quality	40	936	1,101
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		675	721
Ending balance:
Loans acquired with deteriorated credit quality		45	55
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	485	1,682	1,627
Ending balance:
Loans acquired with deteriorated credit quality		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	$ 45	$ 65	$ 64