Loans and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	$ 21,378	[1]	$ 13,976	[1]
60-89 Days Past Due	2,203		3,563
Greater Than 90 Days	2,334		1,647
Total Past Due	25,915		19,186
Total Current	2,102,100		2,057,980
Total	2,128,015		2,077,166		1,793,172
Total 90 Days Past Due Still Accruing	24		97		53
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	4,146	[1]	1,708	[1]
60-89 Days Past Due	307		470
Greater Than 90 Days	283		247
Total Past Due	4,736		2,425
Total Current	508,686		507,184
Total	513,422		509,609
Total 90 Days Past Due Still Accruing	3
Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	641	[1]	467	[1]
60-89 Days Past Due	16		95
Greater Than 90 Days
Total Past Due	657		562
Total Current	54,590		67,744
Total	55,247		68,306
Total 90 Days Past Due Still Accruing
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	14,962	[1]	10,141	[1]
60-89 Days Past Due	1,653		2,711
Greater Than 90 Days	1,907		1,237
Total Past Due	18,522		14,089
Total Current	1,257,042		1,212,734
Total	1,275,564		1,226,823
Total 90 Days Past Due Still Accruing			34
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	1,629	[1]	1,660	[1]
60-89 Days Past Due	227		287
Greater Than 90 Days	144		163
Total Past Due	2,000		2,110
Total Current	281,782		270,318
Total	283,782		272,428
Total 90 Days Past Due Still Accruing	$ 21		$ 63

[1] The Company monitors commercial, agricultural and real estate loans after such loans are 15 days past due. Consumer loans are monitored after such loans are 30 days past due.