Loans Schedule of Loans by Class (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	$ 7,614.5	$ 7,567.7
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group, Mortgage	32.3	46.6
Loans and Leases Receivable, Gross	7,646.8	7,614.3
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	2,821.9	2,822.9
Land acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	364.6	348.7
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	230.6	240.2
Commercial Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	140.8	119.4
Total construction loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	736.0	708.3
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	1,486.6	1,487.4
Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	156.9	158.2
Total real estate loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	5,201.4	5,176.8
Indirect consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	782.9	784.7
Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	173.0	175.1
Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	75.2	74.6
Total consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	1,031.1	1,034.4
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	1,244.1	1,215.4
Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	133.9	136.2
Other, including overdrafts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	4.0	4.9
Loans held for investment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable Held For Investment	7,614.5	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	32.3	46.6
Total loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 7,646.8	$ 7,614.3