Loans and Allowance for Loan Losses - Schedule of Impaired Loans and Related Allowance (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	$ 26,170	$ 23,587
Recorded Investment With No Allowance	2,184	1,818
Recorded Investment With Allowance	19,616	18,157
Total Recorded Investment	21,800	19,975
Related Allowance	6,010	5,953
Average Recorded Investment	24,025	22,348	17,242
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	2,677	3,856
Recorded Investment With No Allowance	20
Recorded Investment With Allowance	2,231	3,450
Total Recorded Investment	2,251	3,450
Related Allowance	1,350	2,092
Average Recorded Investment	2,966	3,801
Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	381	199
Recorded Investment With No Allowance		3
Recorded Investment With Allowance	372	142
Total Recorded Investment	372	145
Related Allowance	131	79
Average Recorded Investment	437	246
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	22,569	19,305
Recorded Investment With No Allowance	2,049	1,786
Recorded Investment With Allowance	16,649	14,407
Total Recorded Investment	18,698	16,193
Related Allowance	4,356	3,708
Average Recorded Investment	20,164	18,068
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	543	227
Recorded Investment With No Allowance	115	29
Recorded Investment With Allowance	364	158
Total Recorded Investment	479	187
Related Allowance	173	74
Average Recorded Investment	$ 458	$ 233