Allowance for Loan Losses - Allowance Related to Unallocated Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Allowance for loan losses:
Beginning balance	$ 15,395	$ 19,405	$ 16,528	$ 19,742
Charge-offs	(456)	(2,600)	(3,440)	(4,465)
Recoveries	506	192	857	920
Provision		300	1,500	1,100
Ending Balance	15,445	17,297	15,445	17,297
Individually evaluated for impairment	1,618		1,618		2,509
Collectively evaluated for impairment	13,827		13,827		14,019
Ending Balance	15,445	17,297	15,445	17,297
Loan balances outstanding:
Individually evaluated for impairment	12,481		12,481		18,057
Collectively evaluated for impairment	874,537		874,537		843,184
Ending Balance	887,018		887,018		861,241
Commercial and Agriculture [Member]
Allowance for loan losses:
Beginning balance	2,067	3,775	2,841	2,811
Charge-offs	(13)	(210)	(326)	(301)
Recoveries	143	62	238	123
Provision	(592)	(576)	(1,148)	418
Ending Balance	1,605	3,051	1,605	3,051
Individually evaluated for impairment	655		655		1,262
Collectively evaluated for impairment	950		950		1,579
Ending Balance	1,605	3,051	1,605	3,051
Loan balances outstanding:
Individually evaluated for impairment	2,319		2,319		3,869
Collectively evaluated for impairment	104,294		104,294		112,006
Ending Balance	106,613		106,613		115,875
Commercial Real Estate [Member]
Allowance for loan losses:
Beginning balance	7,988	9,156	7,559	10,139
Charge-offs	(146)	(635)	(1,720)	(1,266)
Recoveries	251	34	350	257
Provision	(249)	(646)	1,655	(1,221)
Ending Balance	7,844	7,909	7,844	7,909
Individually evaluated for impairment	67		67		445
Collectively evaluated for impairment	7,777		7,777		7,114
Ending Balance	7,844	7,909	7,844	7,909
Loan balances outstanding:
Individually evaluated for impairment	6,638		6,638		10,175
Collectively evaluated for impairment	438,530		438,530		433,671
Ending Balance	445,168		445,168		443,846
Residential Real Estate [Member]
Allowance for loan losses:
Beginning balance	4,439	5,169	5,224	5,780
Charge-offs	(275)	(1,558)	(1,329)	(2,579)
Recoveries	95	63	216	364
Provision	53	1,203	201	1,312
Ending Balance	4,312	4,877	4,312	4,877
Individually evaluated for impairment	896		896		802
Collectively evaluated for impairment	3,416		3,416		4,422
Ending Balance	4,312	4,877	4,312	4,877
Loan balances outstanding:
Individually evaluated for impairment	3,518		3,518		4,005
Collectively evaluated for impairment	262,348		262,348		246,686
Ending Balance	265,866		265,866		250,691
Real Estate Construction [Member]
Allowance for loan losses:
Beginning balance	287	184	184	349
Charge-offs		(136)		(136)
Recoveries	2	1	5	107
Provision	76	111	176	(160)
Ending Balance	365	160	365	160
Collectively evaluated for impairment	365		365		184
Ending Balance	365	160	365	160
Loan balances outstanding:
Collectively evaluated for impairment	55,244		55,244		39,964
Ending Balance	55,244		55,244		39,964
Consumer and Other [Member]
Allowance for loan losses:
Beginning balance	196	149	214	246
Charge-offs	(22)	(61)	(65)	(183)
Recoveries	15	32	48	69
Provision	26	39	18	27
Ending Balance	215	159	215	159
Collectively evaluated for impairment	215		215		214
Ending Balance	215	159	215	159
Loan balances outstanding:
Individually evaluated for impairment	6		6		8
Collectively evaluated for impairment	14,121		14,121		10,857
Ending Balance	14,127		14,127		10,865
Unallocated [Member]
Allowance for loan losses:
Beginning balance	418	972	506	417
Provision	686	169	598	724
Ending Balance	1,104	1,141	1,104	1,141
Collectively evaluated for impairment	1,104		1,104		506
Ending Balance	$ 1,104	$ 1,141	$ 1,104	$ 1,141