Loans - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 73.0	$ 73.0
Provision for Credit Loss Expense	29.2
Loans Charged-Off	(4.6)	(6.0)
Recoveries Collected	1.5
Ending balance	129.1	$ 72.4
Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	8.8
Provision for Credit Loss Expense	2.0
Loans Charged-Off	0.0
Recoveries Collected	0.0
Ending balance	15.7
Owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	10.0
Provision for Credit Loss Expense	2.2
Loans Charged-Off	0.0
Recoveries Collected	0.0
Ending balance	15.7
Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.7
Provision for Credit Loss Expense	0.5
Loans Charged-Off	0.0
Recoveries Collected	0.0
Ending balance	8.1
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	19.5
Provision for Credit Loss Expense	4.7
Loans Charged-Off	0.0
Recoveries Collected	0.0
Ending balance	39.5
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.6
Provision for Credit Loss Expense	0.1
Loans Charged-Off	0.0
Recoveries Collected	0.0
Ending balance	1.1
Commercial and floor plans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	25.5
Provision for Credit Loss Expense	13.5
Loans Charged-Off	(0.8)
Recoveries Collected	0.4
Ending balance	33.5
Commercial purpose secured by 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.9
Provision for Credit Loss Expense	2.4
Loans Charged-Off	(0.1)
Recoveries Collected	0.0
Ending balance	4.4
Land acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.9
Provision for Credit Loss Expense	0.1
Loans Charged-Off	(0.5)
Recoveries Collected	0.0
Ending balance	1.4
Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.5
Provision for Credit Loss Expense	0.5
Loans Charged-Off	0.0
Recoveries Collected	0.0
Ending balance	1.1
Commercial construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.7
Provision for Credit Loss Expense	2.1
Loans Charged-Off	0.0
Recoveries Collected	0.0
Ending balance	6.1
Total construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	6.1
Provision for Credit Loss Expense	2.7
Loans Charged-Off	(0.5)
Recoveries Collected	0.0
Ending balance	8.6
Residential 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.8
Provision for Credit Loss Expense	0.5
Loans Charged-Off	0.0
Recoveries Collected	0.0
Ending balance	12.9
Home equity and HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.0
Provision for Credit Loss Expense	0.1
Loans Charged-Off	0.0
Recoveries Collected	0.0
Ending balance	1.4
Residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.8
Provision for Credit Loss Expense	0.4
Loans Charged-Off	0.0
Recoveries Collected	0.0
Ending balance	14.3
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.5
Provision for Credit Loss Expense	0.3
Loans Charged-Off	0.0
Recoveries Collected	0.0
Ending balance	2.6
Total real estate loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	28.9
Provision for Credit Loss Expense	8.1
Loans Charged-Off	(0.5)
Recoveries Collected	0.0
Ending balance	65.0
Indirect consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.5
Provision for Credit Loss Expense	3.9
Loans Charged-Off	(1.2)
Recoveries Collected	0.7
Ending balance	16.7
Direct and advance lines
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.9
Provision for Credit Loss Expense	0.4
Loans Charged-Off	(1.0)
Recoveries Collected	0.2
Ending balance	5.5
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.5
Provision for Credit Loss Expense	0.4
Loans Charged-Off	(0.8)
Recoveries Collected	0.2
Ending balance	2.6
Total consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	9.9
Provision for Credit Loss Expense	4.7
Loans Charged-Off	(3.0)
Recoveries Collected	1.1
Ending balance	24.8
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.2
Provision for Credit Loss Expense	0.4
Loans Charged-Off	(0.2)
Recoveries Collected	0.0
Ending balance	0.3
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	32.6
Provision for Credit Loss Expense	16.3
Loans Charged-Off	(1.1)
Recoveries Collected	0.4
Ending balance	38.2
Cumulative Effect, Period of Adoption, Adjustment [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	30.0
Cumulative Effect, Period of Adoption, Adjustment [Member] | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.9
Cumulative Effect, Period of Adoption, Adjustment [Member] | Owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3.5
Cumulative Effect, Period of Adoption, Adjustment [Member] | Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	6.9
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	15.3
Cumulative Effect, Period of Adoption, Adjustment [Member] | Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	(0.6)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial and floor plans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	(5.1)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial purpose secured by 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	(3.8)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Land acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	(0.1)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	(0.9)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.3
Cumulative Effect, Period of Adoption, Adjustment [Member] | Total construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.3
Cumulative Effect, Period of Adoption, Adjustment [Member] | Residential 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	10.6
Cumulative Effect, Period of Adoption, Adjustment [Member] | Home equity and HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.5
Cumulative Effect, Period of Adoption, Adjustment [Member] | Residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	11.1
Cumulative Effect, Period of Adoption, Adjustment [Member] | Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.8
Cumulative Effect, Period of Adoption, Adjustment [Member] | Total real estate loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	28.5
Cumulative Effect, Period of Adoption, Adjustment [Member] | Indirect consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	8.8
Cumulative Effect, Period of Adoption, Adjustment [Member] | Direct and advance lines
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3.0
Cumulative Effect, Period of Adoption, Adjustment [Member] | Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.3
Cumulative Effect, Period of Adoption, Adjustment [Member] | Total consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	12.1
Cumulative Effect, Period of Adoption, Adjustment [Member] | Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	(1.1)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ (10.0)