Allowance for Loan Losses - Allowance Related to Unallocated Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Allowance for loan losses:
Beginning balance	$ 19,405	$ 21,931	$ 19,742	$ 21,257
Charge-offs	(2,600)	(2,591)	(4,465)	(6,347)
Recoveries	192	449	920	1,014
Provision	300	1,700	1,100	5,565
Ending Balance	17,297	21,489	17,297	21,489
Individually evaluated for impairment	1,713		1,713		4,006
Collectively evaluated for impairment	15,584		15,584		15,736
Ending Balance	17,297	21,489	17,297	21,489
Loan balances outstanding:
Financing Receivable, Individually evaluated for impairment	17,779		17,779		26,090
Financing Receivable, Collectively evaluated for impairment	801,792		801,792		789,463
Ending Balance	819,571		819,571		815,553
Commercial and Agriculture [Member]
Allowance for loan losses:
Beginning balance	3,775	2,537	2,811	2,876
Charge-offs	(210)	(192)	(301)	(610)
Recoveries	62	180	123	280
Provision	(576)	44	418	23
Ending Balance	3,051	2,569	3,051	2,569
Individually evaluated for impairment	1,054		1,054		286
Collectively evaluated for impairment	1,997		1,997		2,525
Ending Balance	3,051	2,569	3,051	2,569
Loan balances outstanding:
Financing Receivable, Individually evaluated for impairment	3,920		3,920		5,420
Financing Receivable, Collectively evaluated for impairment	101,910		101,910		95,241
Ending Balance	105,830		105,830		100,661
Commercial Real Estate [Member]
Allowance for loan losses:
Beginning balance	9,156	11,701	10,139	10,571
Charge-offs	(635)	(824)	(1,266)	(2,463)
Recoveries	34	196	257	371
Provision	(646)	154	(1,221)	2,748
Ending Balance	7,909	11,227	7,909	11,227
Individually evaluated for impairment	454		454		2,354
Collectively evaluated for impairment	7,455		7,455		7,785
Ending Balance	7,909	11,227	7,909	11,227
Loan balances outstanding:
Financing Receivable, Individually evaluated for impairment	9,813		9,813		13,941
Financing Receivable, Collectively evaluated for impairment	421,920		421,920		420,867
Ending Balance	431,733		431,733		434,808
Residential Real Estate [Member]
Allowance for loan losses:
Beginning balance	5,169	6,379	5,780	5,796
Charge-offs	(1,558)	(1,334)	(2,579)	(2,803)
Recoveries	63	47	364	208
Provision	1,203	867	1,312	2,758
Ending Balance	4,877	5,959	4,877	5,959
Individually evaluated for impairment	204		204		1,199
Collectively evaluated for impairment	4,673		4,673		4,581
Ending Balance	4,877	5,959	4,877	5,959
Loan balances outstanding:
Financing Receivable, Individually evaluated for impairment	4,025		4,025		6,127
Financing Receivable, Collectively evaluated for impairment	240,368		240,368		244,471
Ending Balance	244,393		244,393		250,598
Real Estate Construction [Member]
Allowance for loan losses:
Beginning balance	184	1,095	349	974
Charge-offs	(136)	(192)	(136)	(297)
Recoveries	1	(9)	107	104
Provision	111	171	(160)	284
Ending Balance	160	1,065	160	1,065
Individually evaluated for impairment					107
Collectively evaluated for impairment	160		160		242
Ending Balance	160	1,065	160	1,065
Loan balances outstanding:
Financing Receivable, Individually evaluated for impairment					541
Financing Receivable, Collectively evaluated for impairment	26,661		26,661		19,136
Ending Balance	26,661		26,661		19,677
Consumer [Member]
Allowance for loan losses:
Beginning balance	149	262	246	719
Charge-offs	(61)	(49)	(183)	(174)
Recoveries	32	35	69	51
Provision	39	5	27	(343)
Ending Balance	159	253	159	253
Individually evaluated for impairment	1		1		60
Collectively evaluated for impairment	158		158		186
Ending Balance	159	253	159	253
Loan balances outstanding:
Financing Receivable, Individually evaluated for impairment	21		21		61
Financing Receivable, Collectively evaluated for impairment	10,933		10,933		9,748
Ending Balance	10,954		10,954		9,809
Unallocated [Member]
Allowance for loan losses:
Beginning balance	972	(43)	417	321
Charge-offs
Recoveries
Provision	169	459	724	95
Ending Balance	1,141	416	1,141	416
Individually evaluated for impairment
Collectively evaluated for impairment	1,141		1,141		417
Ending Balance	1,141	416	1,141	416
Loan balances outstanding:
Financing Receivable, Individually evaluated for impairment
Financing Receivable, Collectively evaluated for impairment
Ending Balance