Loans and Allowance for Loan Losses - Schedule of Internal Ratings of Loans Held for Investment (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	$ 2,688,288	$ 2,684,285	$ 2,128,015
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	607,281	596,730
Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	65,121	75,928
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	1,680,807	1,678,514
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	335,079	333,113
Pass [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	2,606,726	2,600,041
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	595,060	584,547
Pass [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	64,755	75,382
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	1,614,094	1,609,242
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	332,817	330,870
Special Mention [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	24,788	24,689
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	5,079	3,032
Special Mention [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	103	245
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	19,043	20,773
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	563	639
Substandard [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	56,624	59,396
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	7,142	9,151
Substandard [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	261	298
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	47,529	48,352
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	1,692	1,595
Doubtful [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	150	159
Doubtful [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	2	3
Doubtful [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	141	147
Doubtful [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	$ 7	$ 9