Loans - Schedule of Contractual Aging of Loans by Portfolio (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Total Loans			$ 8,936.2
Accrued interest reversal	$ 0.2	$ 0.4
Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.5		2.1
Current Loans	69.1		79.5
Nonaccrual Loans	0.0		0.0
Total Loans	70.6		81.6
Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	7.1		7.2
Current Loans	3,573.7		3,467.6
Nonaccrual Loans	13.0		13.0
Total Loans	3,593.8		3,487.8
Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	4.9		10.0
Current Loans	795.0		773.0
Nonaccrual Loans	2.0		1.6
Total Loans	801.9		784.6
Direct and advance lines
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.9		1.8
Current Loans	167.7		176.7
Nonaccrual Loans	0.7		0.5
Total Loans	169.3		179.0
Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	16.5		1.8
Current Loans	268.1		298.9
Nonaccrual Loans	0.7		1.4
Total Loans	285.3		302.1
Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	5.8		2.3
Current Loans	240.4		241.8
Nonaccrual Loans	0.0		0.0
Total Loans	246.2		244.1
Commercial construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.0		0.0
Current Loans	459.3		431.0
Nonaccrual Loans	0.5		0.5
Total Loans	459.8		431.5
Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	22.3		4.1
Current Loans	967.8		971.7
Nonaccrual Loans	1.2		1.9
Total Loans	991.3		977.7
Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	8.2		6.3
Current Loans	1,275.4		1,235.2
Nonaccrual Loans	4.0		4.6
Total Loans	1,287.6		1,246.1
Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.0		1.3
Current Loans	215.8		220.1
Nonaccrual Loans	7.4		5.2
Total Loans	224.2		226.6
Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	38.6		18.9
Current Loans	6,032.7		5,894.6
Nonaccrual Loans	25.6		24.7
Total Loans	6,096.9		5,938.2
Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	7.3		13.9
Current Loans	1,031.8		1,029.2
Nonaccrual Loans	2.7		2.1
Total Loans	1,041.8		1,045.2
Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	15.8		9.7
Current Loans	2,614.3		1,650.3
Nonaccrual Loans	18.5		13.7
Total Loans	2,648.6		1,673.7
Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.8		1.0
Current Loans	276.9		275.7
Nonaccrual Loans	3.1		2.4
Total Loans	282.8		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	64.5		43.5
Current Loans	9,959.4		8,849.8
Nonaccrual Loans	49.9		42.9
Total Loans	10,073.8		8,936.2
30 to 59 Days Past Due | Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.6		0.8
30 to 59 Days Past Due | Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	5.1		5.5
30 to 59 Days Past Due | Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	3.6		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		1.2
30 to 59 Days Past Due | Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	15.8		0.7
30 to 59 Days Past Due | Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	4.7		1.5
30 to 59 Days Past Due | Commercial construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.0		0.0
30 to 59 Days Past Due | Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	20.5		2.2
30 to 59 Days Past Due | Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	4.6		3.8
30 to 59 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	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	30.2		12.3
30 to 59 Days Past Due | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	4.7		9.6
30 to 59 Days Past Due | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	7.1		4.8
30 to 59 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.5		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	43.5		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	1.1		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		0.5
60 to 89 Days Past Due | Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.4		0.8
60 to 89 Days Past Due | Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.6		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	1.0		1.6
60 to 89 Days Past Due | Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	3.3		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	5.5		4.6
60 to 89 Days Past Due | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	1.6		2.9
60 to 89 Days Past Due | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	5.6		2.6
60 to 89 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.6		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	13.3		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		0.8
Equal to or Greater than 90 Days Past Due | Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.8		0.6
Equal to or Greater than 90 Days Past Due | Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.2		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	0.2		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	0.3		0.3
Equal to or Greater than 90 Days Past Due | Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.5		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
Equal to or Greater than 90 Days Past Due | Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.8		0.3
Equal to or Greater than 90 Days Past Due | Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.3		1.1
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 | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	2.9		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	1.0		1.4
Equal to or Greater than 90 Days Past Due | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	3.1		2.3
Equal to or Greater than 90 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Past Due	0.7		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	$ 7.7		$ 5.7