Loans and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	$ 2,077,166	$ 1,775,915
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	509,609	454,087
Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	68,306	68,122
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	1,226,823	1,041,396
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	272,428	212,310
Pass [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	2,010,029	1,688,602
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	498,188	432,110
Pass [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	64,397	65,007
Pass [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	1,176,330	980,308
Pass [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	271,114	211,177
Special Mention [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	17,597	30,798
Special Mention [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	2,193	9,227
Special Mention [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	342	347
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	14,680	20,922
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	382	302
Substandard [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	49,341	56,391
Substandard [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	9,198	12,748
Substandard [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	3,559	2,755
Substandard [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	35,673	40,068
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	911	820
Doubtful [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	199	124
Doubtful [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	30	2
Doubtful [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	8	13
Doubtful [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	140	98
Doubtful [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Internal ratings of loan	$ 21	$ 11