Past Due Aging of Loan Portfolio by Loan Class (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 2,752,527	$ 2,605,479
30-59 Days Past Due	20,233	25,835
60-89 Days Past Due	4,591	11,929
Loans > 90 Days And Accruing	1,974	580
Non- Accrual	56,999	69,592
Total Past Due & Non- Accrual	83,797	107,936
Total Loans	2,836,324	2,713,415
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	574,730	518,764
30-59 Days Past Due	4,800	1,332
60-89 Days Past Due	434	135
Loans > 90 Days And Accruing	46	46
Non- Accrual	12,507	12,246
Total Past Due & Non- Accrual	17,787	13,759
Total Loans	592,517	532,523
Agriculture production financing and other loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	106,782	104,464
30-59 Days Past Due	74	62
60-89 Days Past Due	10
Loans > 90 Days And Accruing	300
Total Past Due & Non- Accrual	384	62
Total Loans	107,166	104,526
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	87,829	69,305
30-59 Days Past Due	341	328
60-89 Days Past Due		3,126
Loans > 90 Days And Accruing		31
Non- Accrual	5,440	8,990
Total Past Due & Non- Accrual	5,781	12,475
Total Loans	93,610	81,780
Commercial and farm land
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,209,194	1,140,897
30-59 Days Past Due	6,931	16,457
60-89 Days Past Due	65	5,783
Loans > 90 Days And Accruing	363
Non- Accrual	24,501	31,093
Total Past Due & Non- Accrual	31,860	53,333
Total Loans	1,241,054	1,194,230
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	453,234	458,925
30-59 Days Past Due	5,835	5,485
60-89 Days Past Due	2,788	2,087
Loans > 90 Days And Accruing	971	191
Non- Accrual	12,444	14,805
Total Past Due & Non- Accrual	22,038	22,568
Total Loans	475,272	481,493
Home Equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	200,402	187,788
30-59 Days Past Due	1,532	1,096
60-89 Days Past Due	899	590
Loans > 90 Days And Accruing	294	261
Non- Accrual	1,761	1,896
Total Past Due & Non- Accrual	4,486	3,843
Total Loans	204,888	191,631
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	76,053	82,837
30-59 Days Past Due	720	1,075
60-89 Days Past Due	395	208
Loans > 90 Days And Accruing		51
Non- Accrual	3	1
Total Past Due & Non- Accrual	1,118	1,335
Total Loans	77,171	84,172
Finance Lease
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,633	3,555
Non- Accrual	337
Total Past Due & Non- Accrual	337
Total Loans	2,970	3,555
Other Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	41,670	38,944
Non- Accrual	6	561
Total Past Due & Non- Accrual	6	561
Total Loans	$ 41,676	$ 39,505