Past Due Aging of Loan Portfolio by Loan Class (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 2,816,679	$ 2,828,813
30-59 Days Past Due	11,828	14,303
60-89 Days Past Due	3,315	3,657
Loans > 90 Days And Accruing	959	2,037
Non- Accrual	46,761	53,399
Total Past Due & Non- Accrual	62,863	73,396
Total Loans	2,879,542	2,902,209
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	625,645	607,442
30-59 Days Past Due	2,058	2,628
60-89 Days Past Due	596	144
Loans > 90 Days And Accruing	191	170
Non- Accrual	9,462	12,195
Total Past Due & Non- Accrual	12,307	15,137
Total Loans	637,952	622,579
Agriculture production financing and other loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	104,198	112,527
30-59 Days Past Due
60-89 Days Past Due
Loans > 90 Days And Accruing
Non- Accrual	86
Total Past Due & Non- Accrual	86
Total Loans	104,284	112,527
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	71,682	93,426
30-59 Days Past Due	373	399
60-89 Days Past Due	36
Non- Accrual	3,786	4,814
Total Past Due & Non- Accrual	4,195	5,213
Total Loans	75,877	98,639
Commercial and farm land
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,258,913	1,238,907
30-59 Days Past Due	1,332	3,276
60-89 Days Past Due	1,063	1,822
Loans > 90 Days And Accruing		65
Non- Accrual	19,303	22,612
Total Past Due & Non- Accrual	21,698	27,775
Total Loans	1,280,611	1,266,682
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	448,486	453,743
30-59 Days Past Due	5,460	5,734
60-89 Days Past Due	1,194	1,338
Loans > 90 Days And Accruing	633	1,246
Non- Accrual	11,856	11,476
Total Past Due & Non- Accrual	19,143	19,794
Total Loans	467,629	473,537
Home Equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	197,003	199,063
30-59 Days Past Due	2,054	1,467
60-89 Days Past Due	335	323
Loans > 90 Days And Accruing	107	556
Non- Accrual	2,268	1,997
Total Past Due & Non- Accrual	4,764	4,343
Total Loans	201,767	203,406
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	72,771	74,919
30-59 Days Past Due	441	799
60-89 Days Past Due	91	30
Loans > 90 Days And Accruing	11
Non- Accrual
Total Past Due & Non- Accrual	543	829
Total Loans	73,314	75,748
Finance Lease
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,886	2,289
30-59 Days Past Due	110
Loans > 90 Days And Accruing	17
Non- Accrual		301
Total Past Due & Non- Accrual	127	301
Total Loans	2,013	2,590
Other Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	36,095	46,497
Non- Accrual		4
Total Past Due & Non- Accrual		4
Total Loans	$ 36,095	$ 46,501