Loans Schedule of Loans by Class (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	$ 7,708.4	$ 7,567.7
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group, Mortgage	50.3	46.6
Loans and Leases Receivable, Gross	7,758.7	7,614.3
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	2,853.3	2,822.9
Land acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	355.6	348.7
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	227.8	240.2
Commercial Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	134.3	119.4
Total construction loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	717.7	708.3
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	1,486.2	1,487.4
Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	175.7	158.2
Total real estate loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	5,232.9	5,176.8
Indirect consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	795.4	784.7
Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	182.5	175.1
Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	77.3	74.6
Total consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	1,055.2	1,034.4
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	1,266.0	1,215.4
Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	151.2	136.2
Other, including overdrafts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	3.1	4.9
Loans held for investment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	7,708.4	7,567.7
Mortgage loans originated for sale
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group, Mortgage	50.3	46.6
Total loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 7,758.7	$ 7,614.3