Loans Held for Investment - Schedule of Contractual Aging of Loans by Portfolio (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Accrued interest reversal	$ 0.0	$ 0.0
Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	1.2		$ 1.6
Nonaccrual Loans	0.0		0.0
Total Loans	64.1		70.2
Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	10.4		12.8
Nonaccrual Loans	8.8		9.6
Total Loans	3,883.2		3,743.2
Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	5.3		8.9
Nonaccrual Loans	1.5		1.9
Total Loans	756.8		805.1
Direct and advance lines
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	1.0		1.2
Nonaccrual Loans	0.2		0.4
Total Loans	132.9		150.6
Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.9		3.7
Nonaccrual Loans	0.5		0.7
Total Loans	260.2		265.0
Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.0		1.9
Nonaccrual Loans	0.0		1.1
Total Loans	268.4		250.9
Commercial construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	1.9		12.2
Nonaccrual Loans	0.0		0.1
Total Loans	610.2		523.5
Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	2.8		17.8
Nonaccrual Loans	0.5		1.9
Total Loans	1,138.8		1,039.4
Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	4.3		6.8
Nonaccrual Loans	4.3		4.6
Total Loans	1,554.9		1,396.3
Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	1.4		2.0
Nonaccrual Loans	5.6		6.2
Total Loans	229.9		220.6
Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	18.9		39.4
Nonaccrual Loans	19.2		22.3
Total Loans	6,806.8		6,399.5
Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	7.5		11.7
Nonaccrual Loans	1.7		2.3
Total Loans	953.8		1,025.9
Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	5.2		9.2
Nonaccrual Loans	7.1		11.8
Total Loans	1,668.7		2,153.9
Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.8		2.4
Nonaccrual Loans	1.9		3.1
Total Loans	212.4		247.6
Other, including overdrafts
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.0		0.0
Nonaccrual Loans	0.0		0.0
Total Loans	1.3		1.6
Loans And Leases Held For Investment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	32.4		62.7
Nonaccrual Loans	29.9		39.5
Total Loans	9,643.0		9,828.5
30 to 59 Days Past Due | Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.6		0.6
30 to 59 Days Past Due | Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	6.1		7.6
30 to 59 Days Past Due | Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	4.0		6.4
30 to 59 Days Past Due | Direct and advance lines
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.7		0.8
30 to 59 Days Past Due | Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.7		2.5
30 to 59 Days Past Due | Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.0		1.5
30 to 59 Days Past Due | Commercial construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	1.9		12.2
30 to 59 Days Past Due | Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	2.6		16.2
30 to 59 Days Past Due | Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.2		4.7
30 to 59 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.0		2.0
30 to 59 Days Past Due | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	8.9		30.5
30 to 59 Days Past Due | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	5.3		7.8
30 to 59 Days Past Due | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	3.7		6.2
30 to 59 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.3		0.4
30 to 59 Days Past Due | Other, including overdrafts
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	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]
Financing Receivable, Loan in Process	18.2		44.9
60 to 89 Days Past Due | Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.2		0.4
60 to 89 Days Past Due | Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	4.0		1.2
60 to 89 Days Past Due | Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	1.1		2.0
60 to 89 Days Past Due | Direct and advance lines
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.2		0.2
60 to 89 Days Past Due | Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.0		1.1
60 to 89 Days Past Due | Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.0		0.4
60 to 89 Days Past Due | Commercial construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.0		0.0
60 to 89 Days Past Due | Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.0		1.5
60 to 89 Days Past Due | Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	2.7		1.6
60 to 89 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.2		0.0
60 to 89 Days Past Due | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	6.9		4.3
60 to 89 Days Past Due | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	1.5		2.6
60 to 89 Days Past Due | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.6		1.8
60 to 89 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.0		0.6
60 to 89 Days Past Due | Other, including overdrafts
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	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]
Financing Receivable, Loan in Process	9.0		9.3
Equal to or Greater than 90 Days Past Due | Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.4		0.6
Equal to or Greater than 90 Days Past Due | Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.3		4.0
Equal to or Greater than 90 Days Past Due | Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	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]
Financing Receivable, Loan in Process	0.1		0.2
Equal to or Greater than 90 Days Past Due | Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.2		0.1
Equal to or Greater than 90 Days Past Due | Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.0		0.0
Equal to or Greater than 90 Days Past Due | Commercial construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	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]
Financing Receivable, Loan in Process	0.2		0.1
Equal to or Greater than 90 Days Past Due | Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	1.4		0.5
Equal to or Greater than 90 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	1.2		0.0
Equal to or Greater than 90 Days Past Due | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	3.1		4.6
Equal to or Greater than 90 Days Past Due | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.7		1.3
Equal to or Greater than 90 Days Past Due | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.9		1.2
Equal to or Greater than 90 Days Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	0.5		1.4
Equal to or Greater than 90 Days Past Due | Other, including overdrafts
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	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]
Financing Receivable, Loan in Process	5.2		8.5
Financial Asset, Not Past Due | Credit card
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	62.9		68.6
Financial Asset, Not Past Due | Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	3,864.0		3,720.8
Financial Asset, Not Past Due | Indirect consumer
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	750.0		794.3
Financial Asset, Not Past Due | Direct and advance lines
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	131.7		149.0
Financial Asset, Not Past Due | Land acquisition & development
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	258.8		260.6
Financial Asset, Not Past Due | Residential construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	268.4		247.9
Financial Asset, Not Past Due | Commercial construction
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	608.3		511.2
Financial Asset, Not Past Due | Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	1,135.5		1,019.7
Financial Asset, Not Past Due | Residential
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	1,546.3		1,384.9
Financial Asset, Not Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	222.9		212.4
Financial Asset, Not Past Due | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	6,768.7		6,337.8
Financial Asset, Not Past Due | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	944.6		1,011.9
Financial Asset, Not Past Due | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	1,656.4		2,132.9
Financial Asset, Not Past Due | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	209.7		242.1
Financial Asset, Not Past Due | Other, including overdrafts
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	1.3		1.6
Financial Asset, Not Past Due | Loans And Leases Held For Investment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Loan in Process	$ 9,580.7		$ 9,726.3