Impaired Loans, by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 14,500	$ 11,399
Recorded Investment With No Allowance	7,714	8,437
Recorded Investment With Allowance	6,786	2,962
Total Recorded Investment	14,500	11,399
Related Allowance	1,744	1,167
Average Recorded Investment	12,770	11,500
Real Estate Loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	14,073	10,659
Recorded Investment With No Allowance	7,385	7,823
Recorded Investment With Allowance	6,688	2,836
Total Recorded Investment	14,073	10,659
Related Allowance	1,687	1,110
Average Recorded Investment	12,278	10,300
Real Estate Loans | Land Development and Construction
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	234	1,134
Recorded Investment With No Allowance	106	992
Recorded Investment With Allowance	128	142
Total Recorded Investment	234	1,134
Related Allowance	117	134
Average Recorded Investment	356	883
Real Estate Loans | Farmland
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	843	641
Recorded Investment With No Allowance	701	492
Recorded Investment With Allowance	142	149
Total Recorded Investment	843	641
Related Allowance	24	24
Average Recorded Investment	700	588
Real Estate Loans | 1-4 Family Mortgages
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	2,525	2,066
Recorded Investment With No Allowance	1,732	1,297
Recorded Investment With Allowance	793	769
Total Recorded Investment	2,525	2,066
Related Allowance	220	216
Average Recorded Investment	2,472	1,933
Real Estate Loans | Commercial Real Estate
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	10,471	6,818
Recorded Investment With No Allowance	4,846	5,042
Recorded Investment With Allowance	5,625	1,776
Total Recorded Investment	10,471	6,818
Related Allowance	1,326	736
Average Recorded Investment	8,750	6,896
Business Loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	179	305
Recorded Investment With No Allowance	81	184
Recorded Investment With Allowance	98	121
Total Recorded Investment	179	305
Related Allowance	57	57
Average Recorded Investment	215	879
Business Loans | Commercial and Industrial Loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	174	284
Recorded Investment With No Allowance	76	163
Recorded Investment With Allowance	98	121
Total Recorded Investment	174	284
Related Allowance	57	57
Average Recorded Investment	206	860
Business Loans | Farm Production and Other Farm Loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	5	21
Recorded Investment With No Allowance	5	21
Total Recorded Investment	5	21
Average Recorded Investment	9	19
Consumer Loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	248	435
Recorded Investment With No Allowance	248	430
Recorded Investment With Allowance		5
Total Recorded Investment	248	435
Average Recorded Investment	277	321
Consumer Loans | Other Consumer Loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	248	435
Recorded Investment With No Allowance	248	430
Recorded Investment With Allowance		5
Total Recorded Investment	248	435
Average Recorded Investment	$ 277	$ 321