Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 2,828,813,000	$ 2,605,479,000
30-59 Days Past Due	14,303,000	25,835,000
60-89 Days Past Due	3,657,000	11,929,000
Loans > 90 Days And Accruing	2,037,000	580,000
Non- Accrual	53,399,000	69,592,000
Total Past Due & Non- Accrual	73,396,000	107,936,000
Total Loans	2,902,209,000	2,713,415,000
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	607,442,000	518,764,000
30-59 Days Past Due	2,628,000	1,332,000
60-89 Days Past Due	144,000	135,000
Loans > 90 Days And Accruing	170,000	46,000
Non- Accrual	12,195,000	12,246,000
Total Past Due & Non- Accrual	15,137,000	13,759,000
Total Loans	622,579,000	532,523,000
Agriculture production financing and other loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	112,527,000	104,464,000
30-59 Days Past Due		62,000
Total Past Due & Non- Accrual		62,000
Total Loans	112,527,000	104,526,000
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	93,426,000	69,305,000
30-59 Days Past Due	399,000	328,000
60-89 Days Past Due		3,126,000
Loans > 90 Days And Accruing		31,000
Non- Accrual	4,814,000	8,990,000
Total Past Due & Non- Accrual	5,213,000	12,475,000
Total Loans	98,639,000	81,780,000
Commercial and farm land
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,238,907,000	1,140,897,000
30-59 Days Past Due	3,276,000	16,457,000
60-89 Days Past Due	1,822,000	5,783,000
Loans > 90 Days And Accruing	65,000
Non- Accrual	22,612,000	31,093,000
Total Past Due & Non- Accrual	27,775,000	53,333,000
Total Loans	1,266,682,000	1,194,230,000
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	453,743,000	458,925,000
30-59 Days Past Due	5,734,000	5,485,000
60-89 Days Past Due	1,338,000	2,087,000
Loans > 90 Days And Accruing	1,246,000	191,000
Non- Accrual	11,476,000	14,805,000
Total Past Due & Non- Accrual	19,794,000	22,568,000
Total Loans	473,537,000	481,493,000
Home Equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	199,063,000	187,788,000
30-59 Days Past Due	1,467,000	1,096,000
60-89 Days Past Due	323,000	590,000
Loans > 90 Days And Accruing	556,000	261,000
Non- Accrual	1,997,000	1,896,000
Total Past Due & Non- Accrual	4,343,000	3,843,000
Total Loans	203,406,000	191,631,000
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	74,919,000	82,837,000
30-59 Days Past Due	799,000	1,075,000
60-89 Days Past Due	30,000	208,000
Loans > 90 Days And Accruing		51,000
Non- Accrual		1,000
Total Past Due & Non- Accrual	829,000	1,335,000
Total Loans	75,748,000	84,172,000
Finance Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,289,000	3,555,000
Non- Accrual	301,000
Total Past Due & Non- Accrual	301,000
Total Loans	2,590,000	3,555,000
Other Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	46,497,000	38,944,000
Non- Accrual	4,000	561,000
Total Past Due & Non- Accrual	4,000	561,000
Total Loans	$ 46,501,000	$ 39,505,000