Past Due Aging of Loan Portfolio by Loan Class (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 2,712,633	$ 2,605,479
30-59 Days Past Due	15,903	25,835
60-89 Days Past Due	5,306	11,929
Loans > 90 Days And Accruing	665	580
Non- Accrual	63,127	69,592
Total Past Due & Non- Accrual	85,001	107,936
Total Loans	2,797,634	2,713,415
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	535,910	518,764
30-59 Days Past Due	2,086	1,332
60-89 Days Past Due	506	135
Loans > 90 Days And Accruing	128	46
Non- Accrual	13,723	12,246
Total Past Due & Non- Accrual	16,443	13,759
Total Loans	552,353	532,523
Agriculture production financing and other loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	106,135	104,464
30-59 Days Past Due		62
Total Past Due & Non- Accrual		62
Total Loans	106,135	104,526
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	91,751	69,305
30-59 Days Past Due	202	328
60-89 Days Past Due		3,126
Loans > 90 Days And Accruing	5	31
Non- Accrual	7,630	8,990
Total Past Due & Non- Accrual	7,837	12,475
Total Loans	99,588	81,780
Commercial and farm land
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,184,882	1,140,897
30-59 Days Past Due	5,095	16,457
60-89 Days Past Due	2,122	5,783
Loans > 90 Days And Accruing	177
Non- Accrual	26,838	31,093
Total Past Due & Non- Accrual	34,232	53,333
Total Loans	1,219,114	1,194,230
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	459,104	458,925
30-59 Days Past Due	6,967	5,485
60-89 Days Past Due	1,936	2,087
Loans > 90 Days And Accruing	327	191
Non- Accrual	12,583	14,805
Total Past Due & Non- Accrual	21,813	22,568
Total Loans	480,917	481,493
Home Equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	203,794	187,788
30-59 Days Past Due	904	1,096
60-89 Days Past Due	669	590
Loans > 90 Days And Accruing	28	261
Non- Accrual	1,855	1,896
Total Past Due & Non- Accrual	3,456	3,843
Total Loans	207,250	191,631
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	83,065	82,837
30-59 Days Past Due	649	1,075
60-89 Days Past Due	73	208
Loans > 90 Days And Accruing		51
Non- Accrual	146	1
Total Past Due & Non- Accrual	868	1,335
Total Loans	83,933	84,172
Finance Lease
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,631	3,555
Non- Accrual	345
Total Past Due & Non- Accrual	345
Total Loans	2,976	3,555
Other Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	45,361	38,944
Non- Accrual	7	561
Total Past Due & Non- Accrual	7	561
Total Loans	$ 45,368	$ 39,505