Loans - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	$ 5.2	$ 2.6	$ 53.7	$ 10.1
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	146.1	74.2	73.0	73.0
Loans Charged-Off	(6.4)	(4.7)	(15.1)	(16.3)
Recoveries Collected	1.8		5.1
Ending balance	145.5	$ 75.0	145.5	$ 75.0
Loans and Leases Receivable, Allowance			52.5
Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	1.5		11.4
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	23.6		8.8
Loans Charged-Off	0.0		0.0
Recoveries Collected	0.1		0.1
Ending balance	25.2		25.2
Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.1		5.7
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	19.1		10.0
Loans Charged-Off	(0.2)		(0.3)
Recoveries Collected	0.1		0.2
Ending balance	19.1		19.1
Multi-family
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	2.3		3.2
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	8.5		0.7
Loans Charged-Off	0.0		0.0
Recoveries Collected	0.0		0.0
Ending balance	10.8		10.8
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	3.9		20.3
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	51.2		19.5
Loans Charged-Off	(0.2)		(0.3)
Recoveries Collected	0.2		0.3
Ending balance	55.1		55.1
Agricultural
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.0		0.0
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.9		1.6
Loans Charged-Off	0.0		(0.1)
Recoveries Collected	0.0		0.0
Ending balance	0.9		0.9
Commercial and floor plans
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	1.2		20.0
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	41.0		25.5
Loans Charged-Off	(3.3)		(4.7)
Recoveries Collected	0.2		1.0
Ending balance	36.7		36.7
Commercial purpose secured by 1-4 family
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.2		2.8
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.1		5.9
Loans Charged-Off	0.0		(0.1)
Recoveries Collected	0.1		0.2
Ending balance	5.0		5.0
Land acquisition & development
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.5		0.3
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.5		1.9
Loans Charged-Off	0.0		(0.5)
Recoveries Collected	0.2		0.2
Ending balance	1.2		1.2
Residential construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.3		1.0
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.3		1.5
Loans Charged-Off	0.0		0.0
Recoveries Collected	0.0		0.0
Ending balance	1.6		1.6
Commercial construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.8		3.1
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	6.3		2.7
Loans Charged-Off	0.0		0.0
Recoveries Collected	0.0		0.0
Ending balance	7.1		7.1
Total construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.6		3.8
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	9.1		6.1
Loans Charged-Off	0.0		(0.5)
Recoveries Collected	0.2		0.2
Ending balance	9.9		9.9
Residential 1-4 family
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	1.1		3.3
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	10.3		1.8
Loans Charged-Off	0.0		0.0
Recoveries Collected	0.0		0.1
Ending balance	9.2		9.2
Home equity and HELOC
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.0		0.1
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.5		1.0
Loans Charged-Off	0.0		0.0
Recoveries Collected	0.0		0.1
Ending balance	1.5		1.5
Residential
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	1.1		3.4
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	11.8		2.8
Loans Charged-Off	0.0		0.0
Recoveries Collected	0.0		0.2
Ending balance	10.7		10.7
Agricultural
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.2		0.6
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3.1		0.5
Loans Charged-Off	0.0		0.0
Recoveries Collected	0.0		0.0
Ending balance	2.9		2.9
Total real estate loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	3.2		21.3
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	75.2		28.9
Loans Charged-Off	(0.2)		(0.8)
Recoveries Collected	0.4		0.7
Ending balance	78.6		78.6
Indirect consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.7		5.1
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	16.4		4.5
Loans Charged-Off	(0.8)		(3.2)
Recoveries Collected	0.7		1.8
Ending balance	17.0		17.0
Direct and advance lines
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.6		1.3
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.1		2.9
Loans Charged-Off	(1.1)		(3.1)
Recoveries Collected	0.2		0.7
Ending balance	4.8		4.8
Credit card
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.6		1.0
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.0		2.5
Loans Charged-Off	(0.6)		(2.2)
Recoveries Collected	0.2		0.6
Ending balance	2.2		2.2
Total consumer loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	1.9		7.4
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	23.5		9.9
Loans Charged-Off	(2.5)		(8.5)
Recoveries Collected	1.1		3.1
Ending balance	24.0		24.0
Credit card
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.3		1.0
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.4		1.2
Loans Charged-Off	(0.4)		(0.9)
Recoveries Collected	0.0		0.1
Ending balance	0.3		0.3
Commercial
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	1.1		23.8
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	46.5		32.6
Loans Charged-Off	(3.7)		(5.7)
Recoveries Collected	0.3		1.3
Ending balance	42.0		42.0
Agriculture Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	0.0
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.9
Loans Charged-Off	0.0
Recoveries Collected	0.0
Ending balance	0.9		0.9
Cumulative Effect, Period of Adoption, Adjustment [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	30.0		30.0
Cumulative Effect, Period of Adoption, Adjustment [Member] | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	4.9		4.9
Cumulative Effect, Period of Adoption, Adjustment [Member] | Owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	3.5		3.5
Cumulative Effect, Period of Adoption, Adjustment [Member] | Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	6.9		6.9
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	15.3		15.3
Cumulative Effect, Period of Adoption, Adjustment [Member] | Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	(0.6)		(0.6)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial and floor plans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	(5.1)		(5.1)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial purpose secured by 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	(3.8)		(3.8)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Land acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	(0.1)		(0.1)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	(0.9)		(0.9)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	1.3		1.3
Cumulative Effect, Period of Adoption, Adjustment [Member] | Total construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	0.3		0.3
Cumulative Effect, Period of Adoption, Adjustment [Member] | Residential 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	10.6		10.6
Cumulative Effect, Period of Adoption, Adjustment [Member] | Home equity and HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	0.5		0.5
Cumulative Effect, Period of Adoption, Adjustment [Member] | Residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	11.1		11.1
Cumulative Effect, Period of Adoption, Adjustment [Member] | Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	1.8		1.8
Cumulative Effect, Period of Adoption, Adjustment [Member] | Total real estate loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	28.5		28.5
Cumulative Effect, Period of Adoption, Adjustment [Member] | Indirect consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	8.8		8.8
Cumulative Effect, Period of Adoption, Adjustment [Member] | Direct and advance lines
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	3.0		3.0
Cumulative Effect, Period of Adoption, Adjustment [Member] | Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	0.3		0.3
Cumulative Effect, Period of Adoption, Adjustment [Member] | Total consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	12.1		12.1
Cumulative Effect, Period of Adoption, Adjustment [Member] | Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	(1.1)		(1.1)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance	$ (10.0)		$ (10.0)