Loans - Schedule of Contractual Aging of Loans by Portfolio (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Total Loans			$ 8,936.2
Accrued interest reversal	$ 0.0	$ 0.3
Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.6	1.6	2.1
Current Loans	68.3	68.3	79.5
Nonaccrual Loans	0.0	0.0	0.0
Total Loans	69.9	69.9	81.6
Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	9.8	9.8	7.2
Current Loans	3,671.2	3,671.2	3,467.6
Nonaccrual Loans	9.9	9.9	13.0
Total Loans	3,690.9	3,690.9	3,487.8
Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	5.7	5.7	10.0
Current Loans	805.3	805.3	773.0
Nonaccrual Loans	1.8	1.8	1.6
Total Loans	812.8	812.8	784.6
Direct and advance lines
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.8	0.8	1.8
Current Loans	160.8	160.8	176.7
Nonaccrual Loans	0.5	0.5	0.5
Total Loans	162.1	162.1	179.0
Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.5	2.5	1.8
Current Loans	271.7	271.7	298.9
Nonaccrual Loans	0.6	0.6	1.4
Total Loans	274.8	274.8	302.1
Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.1	2.1	2.3
Current Loans	225.8	225.8	241.8
Nonaccrual Loans	0.0	0.0	0.0
Total Loans	227.9	227.9	244.1
Commercial construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.1	0.1	0.0
Current Loans	530.2	530.2	431.0
Nonaccrual Loans	0.5	0.5	0.5
Total Loans	530.8	530.8	431.5
Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	4.7	4.7	4.1
Current Loans	1,027.7	1,027.7	971.7
Nonaccrual Loans	1.1	1.1	1.9
Total Loans	1,033.5	1,033.5	977.7
Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	4.2	4.2	6.3
Current Loans	1,302.4	1,302.4	1,235.2
Nonaccrual Loans	4.6	4.6	4.6
Total Loans	1,311.2	1,311.2	1,246.1
Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.1	0.1	1.3
Current Loans	220.0	220.0	220.1
Nonaccrual Loans	7.6	7.6	5.2
Total Loans	227.7	227.7	226.6
Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	18.8	18.8	18.9
Current Loans	6,221.3	6,221.3	5,894.6
Nonaccrual Loans	23.2	23.2	24.7
Total Loans	6,263.3	6,263.3	5,938.2
Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	8.1	8.1	13.9
Current Loans	1,034.4	1,034.4	1,029.2
Nonaccrual Loans	2.3	2.3	2.1
Total Loans	1,044.8	1,044.8	1,045.2
Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	15.9	15.9	9.7
Current Loans	2,567.5	2,567.5	1,650.3
Nonaccrual Loans	16.2	16.2	13.7
Total Loans	2,599.6	2,599.6	1,673.7
Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.9	2.9	1.0
Current Loans	268.7	268.7	275.7
Nonaccrual Loans	3.1	3.1	2.4
Total Loans	274.7	274.7	279.1
Other, including overdrafts
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.0	0.0	0.0
Current Loans	4.2	4.2	0.0
Nonaccrual Loans	0.0	0.0	0.0
Total Loans	4.2	4.2	0.0
Loans And Leases Held For Investment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	45.7	45.7	43.5
Current Loans	10,096.1	10,096.1	8,849.8
Nonaccrual Loans	44.8	44.8	42.9
Total Loans	10,186.6	10,186.6	8,936.2
30 to 59 Days Past Due | Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.7	0.7	0.8
30 to 59 Days Past Due | Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	5.6	5.6	5.5
30 to 59 Days Past Due | Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	4.0	4.0	7.6
30 to 59 Days Past Due | Direct and advance lines
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.5	0.5	1.2
30 to 59 Days Past Due | Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.4	1.4	0.7
30 to 59 Days Past Due | Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.4	0.4	1.5
30 to 59 Days Past Due | Commercial construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.1	0.1	0.0
30 to 59 Days Past Due | Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.9	1.9	2.2
30 to 59 Days Past Due | Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.7	1.7	3.8
30 to 59 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.0	0.0	0.8
30 to 59 Days Past Due | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	9.2	9.2	12.3
30 to 59 Days Past Due | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	5.2	5.2	9.6
30 to 59 Days Past Due | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	5.6	5.6	4.8
30 to 59 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.8	1.8	0.9
30 to 59 Days Past Due | Other, including overdrafts
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.0	0.0	0.0
30 to 59 Days Past Due | Loans And Leases Held For Investment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	21.8	21.8	27.6
60 to 89 Days Past Due | Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.3	0.3	0.5
60 to 89 Days Past Due | Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.6	0.6	1.1
60 to 89 Days Past Due | Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.6	1.6	1.9
60 to 89 Days Past Due | Direct and advance lines
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.2	0.2	0.5
60 to 89 Days Past Due | Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.6	0.6	0.8
60 to 89 Days Past Due | Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.2	0.2	0.8
60 to 89 Days Past Due | Commercial construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.0	0.0	0.0
60 to 89 Days Past Due | Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.8	0.8	1.6
60 to 89 Days Past Due | Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.3	2.3	1.4
60 to 89 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.1	0.1	0.5
60 to 89 Days Past Due | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	3.8	3.8	4.6
60 to 89 Days Past Due | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.1	2.1	2.9
60 to 89 Days Past Due | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	8.2	8.2	2.6
60 to 89 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.2	0.2	0.1
60 to 89 Days Past Due | Other, including overdrafts
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.0	0.0	0.0
60 to 89 Days Past Due | Loans And Leases Held For Investment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	14.3	14.3	10.2
Equal to or Greater than 90 Days Past Due | Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.6	0.6	0.8
Equal to or Greater than 90 Days Past Due | Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	3.6	3.6	0.6
Equal to or Greater than 90 Days Past Due | Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.1	0.1	0.5
Equal to or Greater than 90 Days Past Due | Direct and advance lines
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.1	0.1	0.1
Equal to or Greater than 90 Days Past Due | Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.5	0.5	0.3
Equal to or Greater than 90 Days Past Due | Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.5	1.5	0.0
Equal to or Greater than 90 Days Past Due | Commercial construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.0	0.0	0.0
Equal to or Greater than 90 Days Past Due | Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.0	2.0	0.3
Equal to or Greater than 90 Days Past Due | Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.2	0.2	1.1
Equal to or Greater than 90 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.0	0.0	0.0
Equal to or Greater than 90 Days Past Due | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	5.8	5.8	2.0
Equal to or Greater than 90 Days Past Due | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.8	0.8	1.4
Equal to or Greater than 90 Days Past Due | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.1	2.1	2.3
Equal to or Greater than 90 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.9	0.9	0.0
Equal to or Greater than 90 Days Past Due | Other, including overdrafts
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.0	0.0	0.0
Equal to or Greater than 90 Days Past Due | Loans And Leases Held For Investment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	$ 9.6	$ 9.6	$ 5.7