Loans and Allowance for Loan Losses - Schedule of Impaired Loans and Related Allowance (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	$ 27,447	$ 25,638	$ 26,170
Recorded Investment With No Allowance	2,083	3,549	2,184
Recorded Investment With Allowance	20,426	17,414	19,616
Total Recorded Investment	22,509	20,963	21,800
Related Allowance	5,793	5,357	6,010
Average Recorded Investment	23,163	21,957	24,025
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	1,555	4,218	2,677
Recorded Investment With No Allowance	25	24	20
Recorded Investment With Allowance	1,108	3,641	2,231
Total Recorded Investment	1,133	3,665	2,251
Related Allowance	555	1,990	1,350
Average Recorded Investment	1,232	3,817	2,966
Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	489	948	381
Recorded Investment With No Allowance	25
Recorded Investment With Allowance	448	923	372
Total Recorded Investment	473	923	372
Related Allowance	169	170	131
Average Recorded Investment	483	922	437
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	24,937	20,058	22,569
Recorded Investment With No Allowance	1,956	3,470	2,049
Recorded Investment With Allowance	18,546	12,556	16,649
Total Recorded Investment	20,502	16,026	18,698
Related Allowance	4,901	3,082	4,356
Average Recorded Investment	21,028	16,838	20,164
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Unpaid Contractual Principal Balance	466	414	543
Recorded Investment With No Allowance	77	55	115
Recorded Investment With Allowance	324	294	364
Total Recorded Investment	401	349	479
Related Allowance	168	115	173
Average Recorded Investment	$ 420	$ 380	$ 458