Loans and Allowance for Loan Losses - Schedule of Investment in Loans Related to Balance in Allowance for Loan Losses on Basis of Company's Impairment Methodology (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	$ 70,178	$ 67,137
Loans collectively evaluated for impairment	2,057,837	2,010,029
Total	2,128,015	2,077,166	1,793,172
Commercial [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	11,563	11,421
Loans collectively evaluated for impairment	501,859	498,188
Total	513,422	509,609
Agricultural [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	3,926	3,909
Loans collectively evaluated for impairment	51,321	64,397
Total	55,247	68,306
Real Estate [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	53,452	50,493
Loans collectively evaluated for impairment	1,222,112	1,176,330
Total	1,275,564	1,226,823
Consumer [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	1,237	1,314
Loans collectively evaluated for impairment	282,545	271,114
Total	$ 283,782	$ 272,428