Loans and Allowance for Loan Losses - Schedule of Internal Ratings of Loans Held for Investment (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	$ 2,128,015	$ 2,077,166	$ 1,793,172
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	513,422	509,609
Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	55,247	68,306
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	1,275,564	1,226,823
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	283,782	272,428
Pass [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	2,057,837	2,010,029
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	501,859	498,188
Pass [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	51,321	64,397
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	1,222,112	1,176,330
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	282,545	271,114
Special Mention [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	17,784	17,597
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	2,083	2,193
Special Mention [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	337	342
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	15,015	14,680
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	349	382
Substandard [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	52,182	49,341
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	9,449	9,198
Substandard [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	3,582	3,559
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	38,281	35,673
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	870	911
Doubtful [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	212	199
Doubtful [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	31	30
Doubtful [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	7	8
Doubtful [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	156	140
Doubtful [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	$ 18	$ 21