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, 2014	Dec. 31, 2013	Mar. 31, 2013
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	$ 81,562	$ 84,244
Loan collectively evaluated for impairment	2,606,726	2,600,041
Total	2,688,288	2,684,285	2,128,015
Commercial [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	12,221	12,183
Loan collectively evaluated for impairment	595,060	584,547
Total	607,281	596,730
Agricultural [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	366	546
Loan collectively evaluated for impairment	64,755	75,382
Total	65,121	75,928
Real Estate [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	66,713	69,272
Loan collectively evaluated for impairment	1,614,094	1,609,242
Total	1,680,807	1,678,514
Consumer [Member]
Schedule Of Allowance For Credit Losses And Recorded Investment In Financing Receivables [Line Items]
Loans individually evaluated for impairment	2,262	2,243
Loan collectively evaluated for impairment	332,817	330,870
Total	$ 335,079	$ 333,113