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, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	$ 2,320,883	$ 1,687,052	$ 2,282,524
Individ. evaluated for impairment	65,026	80,299	62,246
Loans pooled for evaluation	2,206,306	1,553,831	2,167,381
Loans acquired with deteriorated credit quality	49,551	52,922	52,897
Beginning balance	36,585	38,245	38,245
Charge-offs	(1,235)	(765)	(2,558)
Recoveries	508	2,197	4,943
(Benefit) provision	197	(1,355)	(4,045)
Ending balance	36,055	38,322	36,585
Ending balance:
Individ. evaluated for impairment	4,692	3,941	4,267
Loans pooled for evaluation	28,265	30,319	28,891
Loans acquired with deteriorated credit quality	3,098	4,062	3,427
Home equity lines of credit [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	345,210	337,802	352,584
Individ. evaluated for impairment	6,290	7,091	6,968
Loans pooled for evaluation	329,961	320,846	336,595
Loans acquired with deteriorated credit quality	8,959	9,865	9,021
Beginning balance	15,676	16,375	16,375
Charge-offs	(341)	(178)	(1,094)
Recoveries	119	329	960
(Benefit) provision	(221)	(160)	(565)
Ending balance	15,233	16,366	15,676
Ending balance:
Individ. evaluated for impairment	1,857	1,394	1,974
Loans pooled for evaluation	12,932	14,489	13,251
Loans acquired with deteriorated credit quality	443	484	451
Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	32,999	27,633	33,074
Individ. evaluated for impairment	338	101	323
Loans pooled for evaluation	32,591	27,457	32,677
Loans acquired with deteriorated credit quality	70	75	74
Beginning balance	719	589	589
Charge-offs	(268)	(127)	(599)
Recoveries	152	183	495
(Benefit) provision	41	(55)	234
Ending balance	644	590	719
Ending balance:
Individ. evaluated for impairment	92	4	142
Loans pooled for evaluation	552	586	572
Loans acquired with deteriorated credit quality			5
Construction [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	40,105	37,394	38,618
Individ. evaluated for impairment	2,655	2,793	2,683
Loans pooled for evaluation	36,714	33,153	35,260
Loans acquired with deteriorated credit quality	736	1,448	675
Beginning balance	1,434	1,559	1,559
Charge-offs		(4)	(4)
Recoveries	11	511	1,377
(Benefit) provision	(695)	(565)	(1,498)
Ending balance	750	1,501	1,434
Ending balance:
Individ. evaluated for impairment	59	104	60
Loans pooled for evaluation	505	769	332
Loans acquired with deteriorated credit quality	186	627	1,042
Construction [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	45,648	22,156	35,518
Individ. evaluated for impairment	94	19	99
Loans pooled for evaluation	45,554	22,081	36,419
Loans acquired with deteriorated credit quality		56
Beginning balance	411	1,263	1,263
Charge-offs		(69)	(69)
Recoveries	19	115	181
(Benefit) provision	(180)	229	(964)
Ending balance	250	1,538	411
Ending balance:
Loans pooled for evaluation	250	1,033	322
Loans acquired with deteriorated credit quality		505	89
RE Mortgage [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	282,383	195,826	279,420
Individ. evaluated for impairment	7,786	7,322	7,188
Loans pooled for evaluation	270,571	184,002	268,227
Loans acquired with deteriorated credit quality	4,026	4,502	4,005
Beginning balance	3,086	3,154	3,154
Charge-offs	(81)	(135)	(171)
Recoveries	1		2
(Benefit) provision	(241)	(39)	101
Ending balance	2,765	2,980	3,086
Ending balance:
Individ. evaluated for impairment	988	666	974
Loans pooled for evaluation	1,709	1,962	1,915
Loans acquired with deteriorated credit quality	69	352	197
RE Mortgage [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	1,364,480	930,472	1,335,939
Individ. evaluated for impairment	44,523	59,613	41,932
Loans pooled for evaluation	1,292,006	840,642	1,263,090
Loans acquired with deteriorated credit quality	27,951	30,217	30,917
Beginning balance	9,227	9,700	9,700
Charge-offs		(13)	(110)
Recoveries	96	172	540
(Benefit) provision	1,128	16	(903)
Ending balance	10,451	9,875	9,227
Ending balance:
Individ. evaluated for impairment	799	968	410
Loans pooled for evaluation	8,464	8,129	8,408
Loans acquired with deteriorated credit quality	1,188	778	409
Home equity loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	32,486	15,776	31,314
Individ. evaluated for impairment	1,359	774	1,278
Loans pooled for evaluation	30,374	14,522	29,266
Loans acquired with deteriorated credit quality	753	480	770
Beginning balance	1,797	1,208	1,208
Charge-offs	(11)		(29)
Recoveries	3	2	34
(Benefit) provision	191	81	584
Ending balance	1,980	1,291	1,797
Ending balance:
Individ. evaluated for impairment	337	178	284
Loans pooled for evaluation	1,643	1,113	1,513
Auto Indirect [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	32	575	112
Individ. evaluated for impairment	14	49	18
Loans pooled for evaluation	18	526	94
Beginning balance	9	66	66
Charge-offs			(3)
Recoveries	20	12	86
(Benefit) provision	(23)	(33)	(140)
Ending balance	6	45	9
Ending balance:
Individ. evaluated for impairment	1	1
Loans pooled for evaluation	5	44	9
C&I [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans	177,540	119,418	174,945
Individ. evaluated for impairment	1,967	2,537	1,757
Loans pooled for evaluation	168,517	110,602	165,753
Loans acquired with deteriorated credit quality	7,056	6,279	7,435
Beginning balance	4,226	4,331	4,331
Charge-offs	(534)	(239)	(479)
Recoveries	87	873	1,268
(Benefit) provision	197	(829)	(894)
Ending balance	3,976	4,136	4,226
Ending balance:
Individ. evaluated for impairment	559	626	423
Loans pooled for evaluation	2,205	2,194	2,569
Loans acquired with deteriorated credit quality	$ 1,212	$ 1,316	$ 1,234