Loans - Schedule of Allowance for Credit Losses for Loans Held for Investment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Interest Receivable	$ 56.7		$ 56.7		$ 46.7
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	146.1	$ 74.2	73.0	$ 73.0
Provision for Credit Loss Expense	5.2	2.6	53.7	10.1
Loans Charged-Off	(6.4)	(4.7)	(15.1)	(16.3)
Recoveries Collected		2.9		8.2
Ending balance	145.5	75.0	145.5	75.0
Financing Receivable, Allowance for Credit Loss, Recovery	1.8		5.1
Real Estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		27.7		31.0
Provision for Credit Loss Expense		0.4		(1.3)
Loans Charged-Off		(0.3)		(3.3)
Recoveries Collected		0.8		2.2
Ending balance		28.6		28.6
Consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		9.1		8.7
Provision for Credit Loss Expense		2.7		7.1
Loans Charged-Off		(2.8)		(9.1)
Recoveries Collected		0.8		3.1
Ending balance		9.8		9.8
Commercial Portfolio Segment [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		35.7		31.3
Provision for Credit Loss Expense		(0.4)		4.3
Loans Charged-Off		(1.6)		(3.5)
Recoveries Collected		1.3		2.9
Ending balance		35.0		35.0
Agricultural
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		1.7		2.0
Provision for Credit Loss Expense		(0.1)		0.0
Loans Charged-Off		0.0		(0.4)
Recoveries Collected		0.0		0.0
Ending balance		$ 1.6		$ 1.6
Commercial Real Estate Non Owner Occupied Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	23.6		8.8
Provision for Credit Loss Expense	1.5		11.4
Loans Charged-Off	0.0		0.0
Ending balance	25.2		25.2
Financing Receivable, Allowance for Credit Loss, Recovery	0.1		0.1
Agricultural Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.9		1.6
Provision for Credit Loss Expense	0.0		0.0
Loans Charged-Off	0.0		(0.1)
Ending balance	0.9		0.9
Financing Receivable, Allowance for Credit Loss, Recovery	0.0		0.0
Commercial and Floor Plans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	41.0		25.5
Provision for Credit Loss Expense	1.2		20.0
Loans Charged-Off	(3.3)		(4.7)
Ending balance	36.7		36.7
Financing Receivable, Allowance for Credit Loss, Recovery	0.2		1.0
Commercial Real Estate Owner Occupied [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	19.1		10.0
Provision for Credit Loss Expense	0.1		5.7
Loans Charged-Off	(0.2)		(0.3)
Ending balance	19.1		19.1
Financing Receivable, Allowance for Credit Loss, Recovery	0.1		0.2
Commercial Real Estate Multi-family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	8.5		0.7
Provision for Credit Loss Expense	2.3		3.2
Loans Charged-Off	0.0		0.0
Ending balance	10.8		10.8
Financing Receivable, Allowance for Credit Loss, Recovery	0.0		0.0
Commercial real estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	51.2		19.5
Provision for Credit Loss Expense	3.9		20.3
Loans Charged-Off	(0.2)		(0.3)
Ending balance	55.1		55.1
Financing Receivable, Allowance for Credit Loss, Recovery	0.2		0.3
Land Acquisition And Development Construction Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.5		1.9
Provision for Credit Loss Expense	0.5		0.3
Loans Charged-Off	0.0		(0.5)
Ending balance	1.2		1.2
Financing Receivable, Allowance for Credit Loss, Recovery	0.2		0.2
Residential construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.3		1.5
Provision for Credit Loss Expense	0.3		1.0
Loans Charged-Off	0.0		0.0
Ending balance	1.6		1.6
Financing Receivable, Allowance for Credit Loss, Recovery	0.0		0.0
Commercial construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	6.3		2.7
Provision for Credit Loss Expense	0.8		3.1
Loans Charged-Off	0.0		0.0
Ending balance	7.1		7.1
Financing Receivable, Allowance for Credit Loss, Recovery	0.0		0.0
Construction Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	9.1		6.1
Provision for Credit Loss Expense	0.6		3.8
Loans Charged-Off	0.0		(0.5)
Ending balance	9.9		9.9
Financing Receivable, Allowance for Credit Loss, Recovery	0.2		0.2
Residential Real Estate 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	10.3		1.8
Provision for Credit Loss Expense	1.1		3.3
Loans Charged-Off	0.0		0.0
Ending balance	9.2		9.2
Financing Receivable, Allowance for Credit Loss, Recovery	0.0		0.1
Home Equity Line of Credit [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.5		1.0
Provision for Credit Loss Expense	0.0		0.1
Loans Charged-Off	0.0		0.0
Ending balance	1.5		1.5
Financing Receivable, Allowance for Credit Loss, Recovery	0.0		0.1
Residential Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	11.8		2.8
Provision for Credit Loss Expense	1.1		3.4
Loans Charged-Off	0.0		0.0
Ending balance	10.7		10.7
Financing Receivable, Allowance for Credit Loss, Recovery	0.0		0.2
Agricultural Real Estate Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3.1		0.5
Provision for Credit Loss Expense	0.2		0.6
Loans Charged-Off	0.0		0.0
Ending balance	2.9		2.9
Financing Receivable, Allowance for Credit Loss, Recovery	0.0		0.0
Real Estate Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	75.2		28.9
Provision for Credit Loss Expense	3.2		21.3
Loans Charged-Off	(0.2)		(0.8)
Ending balance	78.6		78.6
Financing Receivable, Allowance for Credit Loss, Recovery	0.4		0.7
Consumer Indirect Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	16.4		4.5
Provision for Credit Loss Expense	0.7		5.1
Loans Charged-Off	(0.8)		(3.2)
Ending balance	17.0		17.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.7		1.8
Direct consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.1		2.9
Provision for Credit Loss Expense	0.6		1.3
Loans Charged-Off	(1.1)		(3.1)
Ending balance	4.8		4.8
Financing Receivable, Allowance for Credit Loss, Recovery	0.2		0.7
Credit Card Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.0		2.5
Provision for Credit Loss Expense	0.6		1.0
Loans Charged-Off	(0.6)		(2.2)
Ending balance	2.2		2.2
Financing Receivable, Allowance for Credit Loss, Recovery	0.2		0.6
Consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	23.5		9.9
Provision for Credit Loss Expense	1.9		7.4
Loans Charged-Off	(2.5)		(8.5)
Ending balance	24.0		24.0
Financing Receivable, Allowance for Credit Loss, Recovery	1.1		3.1
Commercial Purpose secured by 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.1		5.9
Provision for Credit Loss Expense	0.2		2.8
Loans Charged-Off	0.0		(0.1)
Ending balance	5.0		5.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.1		0.2
Commercial Borrower [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.4		1.2
Provision for Credit Loss Expense	0.3		1.0
Loans Charged-Off	(0.4)		(0.9)
Ending balance	0.3		0.3
Financing Receivable, Allowance for Credit Loss, Recovery	0.0		0.1
Commercial Portfolio Segment [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	46.5		32.6
Provision for Credit Loss Expense	1.1		23.8
Loans Charged-Off	(3.7)		(5.7)
Ending balance	42.0		42.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.3		1.3
Cumulative Effect, Period of Adoption, Adjustment [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	30.0		30.0
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial Real Estate Non Owner Occupied Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	4.9		4.9
Cumulative Effect, Period of Adoption, Adjustment [Member] | Agricultural Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	(0.6)		(0.6)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial and Floor Plans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	(5.1)		(5.1)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial Real Estate Owner Occupied [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	3.5		3.5
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial Real Estate Multi-family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	6.9		6.9
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial real estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	15.3		15.3
Cumulative Effect, Period of Adoption, Adjustment [Member] | Land Acquisition And Development Construction Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	(0.1)		(0.1)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Residential construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	(0.9)		(0.9)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	1.3		1.3
Cumulative Effect, Period of Adoption, Adjustment [Member] | Construction Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	0.3		0.3
Cumulative Effect, Period of Adoption, Adjustment [Member] | Residential Real Estate 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	10.6		10.6
Cumulative Effect, Period of Adoption, Adjustment [Member] | Home Equity Line of Credit [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	0.5		0.5
Cumulative Effect, Period of Adoption, Adjustment [Member] | Residential Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	11.1		11.1
Cumulative Effect, Period of Adoption, Adjustment [Member] | Agricultural Real Estate Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	1.8		1.8
Cumulative Effect, Period of Adoption, Adjustment [Member] | Real Estate Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	28.5		28.5
Cumulative Effect, Period of Adoption, Adjustment [Member] | Consumer Indirect Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	8.8		8.8
Cumulative Effect, Period of Adoption, Adjustment [Member] | Direct consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	3.0		3.0
Cumulative Effect, Period of Adoption, Adjustment [Member] | Credit Card Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	0.3		0.3
Cumulative Effect, Period of Adoption, Adjustment [Member] | Consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	12.1		12.1
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial Purpose secured by 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	(3.8)		(3.8)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial Borrower [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	(1.1)		(1.1)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial Portfolio Segment [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	(10.0)		(10.0)
Loans And Leases Held For Investment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Interest Receivable	$ 45.7		$ 45.7