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	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	$ 79,930	$ 84,244
Loans collectively evaluated for impairment	2,693,144	2,600,041
Total	2,773,074	2,684,285	2,560,890
Agricultural [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	399	546
Loans collectively evaluated for impairment	74,734	75,382
Total	75,133	75,928
Real Estate [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	64,760	69,272
Loans collectively evaluated for impairment	1,668,844	1,609,242
Total	1,733,604	1,678,514
Commercial [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	12,762	12,183
Loans collectively evaluated for impairment	605,163	584,547
Total	617,925	596,730
Consumer [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	2,009	2,243
Loans collectively evaluated for impairment	344,403	330,870
Total	$ 346,412	$ 333,113