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	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	$ 67,137	$ 87,313
Loan collectively evaluated for impairment	2,010,029	1,688,602
Total	2,077,166	1,775,915
Commercial [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	11,421	21,977
Loan collectively evaluated for impairment	498,188	432,110
Total	509,609	454,087
Agricultural [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	3,909	3,115
Loan collectively evaluated for impairment	64,397	65,007
Total	68,306	68,122
Real Estate [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	50,493	61,088
Loan collectively evaluated for impairment	1,176,330	980,308
Total	1,226,823	1,041,396
Consumer [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	1,314	1,133
Loan collectively evaluated for impairment	271,114	211,177
Total	$ 272,428	$ 212,310