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, 2014	Dec. 31, 2013
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	$ 72,982	$ 84,244
Loan collectively evaluated for impairment	2,856,206	2,600,041
Total	2,929,188	2,684,285
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	13,688	12,183
Loan collectively evaluated for impairment	626,266	584,547
Total	639,954	596,730
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	1,733	2,243
Loan collectively evaluated for impairment	358,953	330,870
Total	360,686	333,113
Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	593	546
Loan collectively evaluated for impairment	105,101	75,382
Total	105,694	75,928
Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	56,968	69,272
Loan collectively evaluated for impairment	1,765,886	1,609,242
Total	$ 1,822,854	$ 1,678,514