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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	$ 2,469,566	$ 1,765,871	$ 2,469,566	$ 1,765,871	$ 2,282,524
Individ. evaluated for impairment	63,474	70,046	63,474	70,046	62,246
Loans pooled for evaluation	2,361,149	1,648,036	2,361,149	1,648,036	2,167,381
Loans acquired with deteriorated credit quality	44,943	47,789	44,943	47,789	52,897
Beginning balance	35,455	39,968	36,585	38,245	38,245
Charge-offs	(687)	(345)	(2,436)	(2,139)	(2,558)
Recoveries	2,616	1,274	3,671	4,438	4,943
(Benefit) provision	(866)	(2,977)	(1,302)	(2,624)	(4,045)
Ending balance	36,518	37,920	36,518	37,920	36,585
Ending balance:
Individ. evaluated for impairment	2,534	4,091	2,534	4,091	4,267
Loans pooled for evaluation	30,145	30,420	30,145	30,420	28,891
Home equity lines of credit [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	331,689	321,942	331,689	321,942	352,584
Individ. evaluated for impairment	6,212	6,733	6,212	6,733	6,968
Loans pooled for evaluation	316,910	306,006	316,910	306,006	336,595
Loans acquired with deteriorated credit quality	8,567	9,203	8,567	9,203	9,021
Beginning balance	13,993	18,055	15,676	16,375	16,375
Charge-offs	(199)	(136)	(624)	(991)	(1,094)
Recoveries	197	249	546	758	960
(Benefit) provision	(1,202)	(1,239)	(2,809)	787	(565)
Ending balance	12,789	16,929	12,789	16,929	15,676
Ending balance:
Individ. evaluated for impairment	937	2,001	937	2,001	1,974
Loans pooled for evaluation	11,438	14,428	11,438	14,428	13,251
Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	33,027	29,088	33,027	29,088	33,074
Individ. evaluated for impairment	293	194	293	194	323
Loans pooled for evaluation	32,670	28,830	32,670	28,830	32,677
Loans acquired with deteriorated credit quality	64	64	64	64	74
Beginning balance	705	556	719	589	589
Charge-offs	(348)	(118)	(792)	(389)	(599)
Recoveries	122	71	381	373	495
(Benefit) provision	189	160	360	96	234
Ending balance	668	669	668	669	719
Ending balance:
Individ. evaluated for impairment	85	69	85	69	142
Loans pooled for evaluation	583	600	583	600	572
Construction [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	46,067	24,386	46,067	24,386	38,618
Individ. evaluated for impairment	318	2,720	318	2,720	2,683
Loans pooled for evaluation	45,019	20,140	45,019	20,140	35,260
Loans acquired with deteriorated credit quality	730	1,526	730	1,526	675
Beginning balance	842	1,511	1,434	1,559	1,559
Charge-offs				(4)	(4)
Recoveries	1,717	769	1,728	1,377	1,377
(Benefit) provision	(1,523)	(702)	(2,126)	(1,354)	(1,498)
Ending balance	1,036	1,578	1,036	1,578	1,434
Ending balance:
Individ. evaluated for impairment		60		60	60
Loans pooled for evaluation	859	349	859	349	332
Construction [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	64,006	18,627	64,006	18,627	36,518
Individ. evaluated for impairment	80	114	80	114	99
Loans pooled for evaluation	63,926	18,439	63,926	18,439	36,419
Loans acquired with deteriorated credit quality	0	74	0	74
Beginning balance	409	1,337	411	1,263	1,263
Charge-offs				(69)	(69)
Recoveries	19	26	52	161	181
(Benefit) provision	235	(964)	200	(956)	(964)
Ending balance	663	399	663	399	411
Ending balance:
Loans pooled for evaluation	663	295	663	295	322
RE Mortgage [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	302,105	228,407	302,105	228,407	279,420
Individ. evaluated for impairment	7,286	7,142	7,286	7,142	7,188
Loans pooled for evaluation	292,607	217,478	292,607	217,478	268,227
Loans acquired with deteriorated credit quality	2,212	3,787	2,212	3,787	4,005
Beginning balance	2,835	2,832	3,086	3,154	3,154
Charge-offs	(15)	(31)	(224)	(167)	(171)
Recoveries	60		61		2
(Benefit) provision	(241)	51	(284)	(135)	101
Ending balance	2,639	2,852	2,639	2,852	3,086
Ending balance:
Individ. evaluated for impairment	345	902	345	902	974
Loans pooled for evaluation	2,178	1,765	2,178	1,765	1,915
RE Mortgage [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	1,454,977	985,746	1,454,977	985,746	1,335,939
Individ. evaluated for impairment	46,237	50,322	46,237	50,322	41,932
Loans pooled for evaluation	1,381,982	908,725	1,381,982	908,725	1,263,090
Loans acquired with deteriorated credit quality	26,758	26,699	26,758	26,699	30,917
Beginning balance	10,141	9,831	9,227	9,700	9,700
Charge-offs		(49)		(107)	(110)
Recoveries	78	42	227	513	540
(Benefit) provision	882	(53)	1,647	(335)	(903)
Ending balance	11,101	9,771	11,101	9,771	9,227
Ending balance:
Individ. evaluated for impairment	502	413	502	413	410
Loans pooled for evaluation	8,659	9,182	8,659	9,182	8,408
Home equity loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	38,365	22,378	38,365	22,378	31,314
Individ. evaluated for impairment	1,459	896	1,459	896	1,278
Loans pooled for evaluation	35,287	20,866	35,287	20,866	29,266
Loans acquired with deteriorated credit quality	1,619	616	1,619	616	770
Beginning balance	2,128	1,411	1,797	1,208	1,208
Charge-offs	(73)		(201)	(11)	(29)
Recoveries	235	4	244	31	34
(Benefit) provision	622	229	1,072	416	584
Ending balance	2,912	1,644	2,912	1,644	1,797
Ending balance:
Individ. evaluated for impairment	218	213	218	213	284
Loans pooled for evaluation	2,694	1,431	2,694	1,431	1,513
Auto Indirect [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans		212		212	112
Individ. evaluated for impairment		23		23	18
Loans pooled for evaluation		189		189	94
Beginning balance		28	9	66	66
Charge-offs			(4)		(3)
Recoveries	2	19	38	70	86
(Benefit) provision	(2)	(31)	(43)	(120)	(140)
Ending balance		16		16	9
Ending balance:
Loans pooled for evaluation		16		16	9
C&I [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	199,330	135,085	199,330	135,085	174,945
Individ. evaluated for impairment	1,589	1,902	1,589	1,902	1,757
Loans pooled for evaluation	192,748	127,363	192,748	127,363	165,753
Loans acquired with deteriorated credit quality	4,993	5,820	4,993	5,820	7,435
Beginning balance	4,402	4,407	4,226	4,331	4,331
Charge-offs	(52)	(11)	(591)	(401)	(479)
Recoveries	186	94	394	1,155	1,268
(Benefit) provision	174	(428)	681	(1,023)	(894)
Ending balance	4,710	4,062	4,710	4,062	4,226
Ending balance:
Individ. evaluated for impairment	447	433	447	433	423
Loans pooled for evaluation	3,071	2,354	3,071	2,354	2,569
Receivables Acquired with Deteriorated Credit Quality [Member]
Ending balance:
Loans acquired with deteriorated credit quality	3,839	3,409	3,839	3,409	3,427
Receivables Acquired with Deteriorated Credit Quality [Member] | Home equity lines of credit [Member]
Ending balance:
Loans acquired with deteriorated credit quality	413	500	413	500	451
Receivables Acquired with Deteriorated Credit Quality [Member] | Other Consumer [Member]
Ending balance:
Loans acquired with deteriorated credit quality					5
Receivables Acquired with Deteriorated Credit Quality [Member] | Construction [Member] | Residential [Member]
Ending balance:
Loans acquired with deteriorated credit quality	177	1,169	177	1,169	1,042
Receivables Acquired with Deteriorated Credit Quality [Member] | Construction [Member] | Commercial [Member]
Ending balance:
Loans acquired with deteriorated credit quality		104		104	89
Receivables Acquired with Deteriorated Credit Quality [Member] | RE Mortgage [Member] | Residential [Member]
Ending balance:
Loans acquired with deteriorated credit quality	115	185	115	185	197
Receivables Acquired with Deteriorated Credit Quality [Member] | RE Mortgage [Member] | Commercial [Member]
Ending balance:
Loans acquired with deteriorated credit quality	1,942	176	1,942	176	409
Receivables Acquired with Deteriorated Credit Quality [Member] | C&I [Member]
Ending balance:
Loans acquired with deteriorated credit quality	$ 1,192	$ 1,275	$ 1,192	$ 1,275	$ 1,234