Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 3,703,026	$ 3,549,125
30-59 Days Past Due	12,541	16,864
60-89 Days Past Due	6,965	8,668
Loans 90 Days And Accruing	831	1,350
Non-Accrual	49,104	56,402
Total Past Due & Non-Accrual	69,441	83,284
Total	3,772,467	3,632,409
Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	887,365	749,020
30-59 Days Past Due	2,522	2,628
60-89 Days Past Due	3,758	774
Loans 90 Days And Accruing	186
Non-Accrual	7,139	9,283
Total Past Due & Non-Accrual	13,605	12,685
Total	900,970	761,705
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	88,526	114,305
30-59 Days Past Due	174	13
60-89 Days Past Due	280
Non-Accrual	10,669	30
Total Past Due & Non-Accrual	11,123	43
Total	99,649	114,348
Real estate loans - Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	176,229	171,046
30-59 Days Past Due	1,033	1,058
Loans 90 Days And Accruing
Non-Accrual	951	4,978
Total Past Due & Non-Accrual	1,984	6,036
Total	178,213	177,082
Real estate loans - Commercial and farm land
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,583,226	1,573,403
30-59 Days Past Due	4,044	3,807
60-89 Days Past Due	1,748	5,801
Loans 90 Days And Accruing		703
Non-Accrual	14,680	28,095
Total Past Due & Non-Accrual	20,472	38,406
Total	1,603,698	1,611,809
Real estate loans - Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	607,204	595,192
30-59 Days Past Due	3,841	7,156
60-89 Days Past Due	469	1,475
Loans 90 Days And Accruing	549	494
Non-Accrual	13,546	12,068
Total Past Due & Non-Accrual	18,405	21,193
Total	625,609	616,385
Real estate loans - Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	266,566	251,188
30-59 Days Past Due	710	1,652
60-89 Days Past Due	703	563
Loans 90 Days And Accruing	77	153
Non-Accrual	1,896	1,667
Total Past Due & Non-Accrual	3,386	4,035
Total	269,952	255,223
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	66,366	69,061
30-59 Days Past Due	217	550
60-89 Days Past Due	7	55
Loans 90 Days And Accruing	19
Non-Accrual	223	117
Total Past Due & Non-Accrual	466	722
Total	66,832	69,783
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,208	1,545
Total	1,208	1,545
Other loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	26,336	24,365
30-59 Days Past Due
Non-Accrual		164
Total Past Due & Non-Accrual		164
Total	$ 26,336	$ 24,529