Loans - Schedule of Contractual Aging of Loans by Portfolio (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	$ 8,918.0		$ 8,930.7
Accrued interest reversal	0.2	$ 0.6
Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.0		2.1
Current Loans	73.9		79.5
Nonaccrual Loans	0.0		0.0
Total Loans	75.9		81.6
Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	9.3		7.2
Current Loans	3,519.9		3,467.6
Nonaccrual Loans	14.2		13.0
Total Loans	3,543.4		3,487.8
Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	10.0		10.0
Current Loans	762.6		773.0
Nonaccrual Loans	2.0		1.6
Total Loans	774.6		784.6
Direct and advance lines
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.9		1.8
Current Loans	172.5		176.7
Nonaccrual Loans	0.6		0.5
Total Loans	175.0		179.0
Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.9		1.8
Current Loans	286.9		298.9
Nonaccrual Loans	0.7		1.4
Total Loans	290.5		302.1
Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	3.7		2.3
Current Loans	224.3		241.8
Nonaccrual Loans	0.4		0.0
Total Loans	228.4		244.1
Commercial construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.1		0.0
Current Loans	458.8		431.0
Nonaccrual Loans	0.5		0.5
Total Loans	459.4		431.5
Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	6.7		4.1
Current Loans	970.0		971.7
Nonaccrual Loans	1.6		1.9
Total Loans	978.3		977.7
Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	3.3		6.3
Current Loans	1,222.9		1,235.2
Nonaccrual Loans	4.8		4.6
Total Loans	1,231.0		1,246.1
Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	3.7		1.3
Current Loans	213.3		220.1
Nonaccrual Loans	7.6		5.2
Total Loans	224.6		226.6
Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	23.0		18.9
Current Loans	5,926.1		5,894.6
Nonaccrual Loans	28.2		24.7
Total Loans	5,977.3		5,938.2
Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	13.9		13.9
Current Loans	1,009.0		1,029.2
Nonaccrual Loans	2.6		2.1
Total Loans	1,025.5		1,045.2
Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	23.5		9.7
Current Loans	1,619.5		1,650.3
Nonaccrual Loans	17.0		13.7
Total Loans	1,660.0		1,673.7
Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	4.7		1.0
Current Loans	249.9		275.7
Nonaccrual Loans	3.3		2.4
Total Loans	257.9		279.1
Other, including overdrafts
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.0		0.0
Current Loans	3.7		0.0
Nonaccrual Loans	0.0		0.0
Total Loans	3.7		0.0
Loans And Leases Held For Investment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	65.1		43.5
Current Loans	8,808.2		8,849.8
Nonaccrual Loans	51.1		42.9
Total Loans	8,924.4		8,936.2
30 to 59 Days Past Due | Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.8		0.8
30 to 59 Days Past Due | Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	7.1		5.5
30 to 59 Days Past Due | Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	7.3		7.6
30 to 59 Days Past Due | Direct and advance lines
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.3		1.2
30 to 59 Days Past Due | Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.2		0.7
30 to 59 Days Past Due | Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	3.0		1.5
30 to 59 Days Past Due | Commercial construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.1		0.0
30 to 59 Days Past Due | Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	5.3		2.2
30 to 59 Days Past Due | Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.3		3.8
30 to 59 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.8		0.8
30 to 59 Days Past Due | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	15.5		12.3
30 to 59 Days Past Due | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	9.4		9.6
30 to 59 Days Past Due | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	12.6		4.8
30 to 59 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.9		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
30 to 59 Days Past Due | Loans And Leases Held For Investment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	40.4		27.6
60 to 89 Days Past Due | Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.3		0.5
60 to 89 Days Past Due | Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.2		1.1
60 to 89 Days Past Due | Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.2		1.9
60 to 89 Days Past Due | Direct and advance lines
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.5		0.5
60 to 89 Days Past Due | Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.3		0.8
60 to 89 Days Past Due | Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.0		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
60 to 89 Days Past Due | Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.3		1.6
60 to 89 Days Past Due | Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.8		1.4
60 to 89 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.0		0.5
60 to 89 Days Past Due | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.3		4.6
60 to 89 Days Past Due | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	3.0		2.9
60 to 89 Days Past Due | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	6.6		2.6
60 to 89 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.8		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
60 to 89 Days Past Due | Loans And Leases Held For Investment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	12.7		10.2
Equal to or Greater than 90 Days Past Due | Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.9		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	1.0		0.6
Equal to or Greater than 90 Days Past Due | Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.5		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
Equal to or Greater than 90 Days Past Due | Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.4		0.3
Equal to or Greater than 90 Days Past Due | Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.7		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
Equal to or Greater than 90 Days Past Due | Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.1		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		1.1
Equal to or Greater than 90 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.9		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	5.2		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	1.5		1.4
Equal to or Greater than 90 Days Past Due | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	4.3		2.3
Equal to or Greater than 90 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.0		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
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	$ 12.0		$ 5.7