Loans and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	$ 13,976	[1]	$ 13,485	[1]
60-89 Days Past Due	3,563		1,165
Greater Than 90 Days	1,647		1,035
Total Past Due	19,186		15,685
Total Current	2,057,980		1,760,230
Total	2,077,166		1,775,915
Total 90 Days Past Due Still Accruing	97		96
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	1,708	[1]	1,574	[1]
60-89 Days Past Due	470		430
Greater Than 90 Days	247
Total Past Due	2,425		2,004
Total Current	507,184		452,083
Total	509,609		454,087
Total 90 Days Past Due Still Accruing
Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	467	[1]	300	[1]
60-89 Days Past Due	95		60
Greater Than 90 Days
Total Past Due	562		360
Total Current	67,744		67,762
Total	68,306		68,122
Total 90 Days Past Due Still Accruing
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	10,141	[1]	10,215	[1]
60-89 Days Past Due	2,711		547
Greater Than 90 Days	1,237		988
Total Past Due	14,089		11,750
Total Current	1,212,734		1,029,646
Total	1,226,823		1,041,396
Total 90 Days Past Due Still Accruing	34		62
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
15-59 Days Past Due	1,660	[1]	1,396	[1]
60-89 Days Past Due	287		128
Greater Than 90 Days	163		47
Total Past Due	2,110		1,571
Total Current	270,318		210,739
Total	272,428		212,310
Total 90 Days Past Due Still Accruing	$ 63		$ 34

[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.