Loans Held for Investment - Schedule of Allowance for Credit Losses for Loans Held for Investment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 122.3	$ 144.3
Provision charged to operating expense	61.3	(5.1)
Loans Charged-Off	(19.9)	(4.9)
Ending balance	247.2	136.6
Financing Receivable, Allowance for Credit Loss, Recovery	3.2	2.0
Commercial Real Estate Non Owner Occupied Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	17.3	25.5
Provision charged to operating expense	6.7	(2.5)
Loans Charged-Off	(2.9)	0.0
Ending balance	45.2	23.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Agricultural Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.3	0.7
Provision charged to operating expense	(0.5)	(0.1)
Loans Charged-Off	(0.2)	0.0
Ending balance	4.1	0.6
Financing Receivable, Allowance for Credit Loss, Recovery	1.1	0.0
Commercial and Floor Plans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	27.1	34.2
Provision charged to operating expense	18.4	(1.8)
Loans Charged-Off	(4.2)	(1.8)
Ending balance	52.9	31.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.4	0.4
Commercial Real Estate Owner Occupied [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13.3	18.3
Provision charged to operating expense	4.6	(0.8)
Loans Charged-Off	(2.2)	(0.1)
Ending balance	25.2	17.4
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Commercial Real Estate Multi-family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13.3	11.0
Provision charged to operating expense	11.4	0.8
Loans Charged-Off	0.0	0.0
Ending balance	54.6	11.8
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Commercial real estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	43.9	54.8
Provision charged to operating expense	22.7	(2.5)
Loans Charged-Off	(5.1)	(0.1)
Ending balance	125.0	52.2
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Land Acquisition And Development Construction Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.5	1.3
Provision charged to operating expense	(0.8)	(0.2)
Loans Charged-Off	(2.7)	0.0
Ending balance	0.5	1.2
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.1
Residential construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.4	1.6
Provision charged to operating expense	0.9	(0.2)
Loans Charged-Off	0.0	0.0
Ending balance	3.3	1.4
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Commercial construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	6.0	7.3
Provision charged to operating expense	4.0	(0.2)
Loans Charged-Off	0.0	0.0
Ending balance	10.2	7.2
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.1
Construction Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	8.9	10.2
Provision charged to operating expense	4.1	(0.6)
Loans Charged-Off	(2.7)	0.0
Ending balance	14.0	9.8
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.2
Residential Real Estate 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13.4	11.4
Provision charged to operating expense	5.2	0.1
Loans Charged-Off	(0.1)	0.0
Ending balance	18.7	11.5
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.0
Home Equity Line of Credit [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.2	1.4
Provision charged to operating expense	0.1	0.0
Loans Charged-Off	0.0	(0.1)
Ending balance	1.4	1.4
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.1
Residential Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	14.6	12.8
Provision charged to operating expense	5.3	0.1
Loans Charged-Off	(0.1)	(0.1)
Ending balance	20.1	12.9
Financing Receivable, Allowance for Credit Loss, Recovery	0.2	0.1
Agricultural Real Estate Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.9	2.7
Provision charged to operating expense	6.4	0.1
Loans Charged-Off	(5.2)	0.0
Ending balance	5.4	2.8
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	69.3	80.5
Provision charged to operating expense	38.5	(2.9)
Loans Charged-Off	(13.1)	(0.2)
Ending balance	164.5	77.7
Financing Receivable, Allowance for Credit Loss, Recovery	0.3	0.3
Consumer Indirect Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	14.3	16.7
Provision charged to operating expense	(0.3)	0.0
Loans Charged-Off	(0.9)	(1.3)
Ending balance	13.5	16.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.4	0.6
Direct consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.6	4.6
Provision charged to operating expense	(0.1)	0.7
Loans Charged-Off	(0.8)	(0.8)
Ending balance	4.6	4.8
Financing Receivable, Allowance for Credit Loss, Recovery	0.9	0.3
Credit Card Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.2	2.6
Provision charged to operating expense	0.6	(0.7)
Loans Charged-Off	(0.6)	(0.6)
Ending balance	2.3	1.5
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.2
Consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	21.1	23.9
Provision charged to operating expense	0.2	0.0
Loans Charged-Off	(2.3)	(2.7)
Ending balance	20.4	22.3
Financing Receivable, Allowance for Credit Loss, Recovery	1.4	1.1
Commercial Purpose secured by 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.4	4.7
Provision charged to operating expense	0.4	(0.2)
Loans Charged-Off	0.0	0.0
Ending balance	5.0	4.7
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.2
Commercial Borrower [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.1	0.3
Provision charged to operating expense	0.3	0.2
Loans Charged-Off	(0.1)	(0.2)
Ending balance	0.3	0.3
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Commercial Portfolio Segment [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	31.6	39.2
Provision charged to operating expense	19.1	(1.8)
Loans Charged-Off	(4.3)	(2.0)
Ending balance	58.2	36.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.4	0.6
Agriculture Portfolio Segment [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.3	0.7
Provision charged to operating expense	(0.5)	(0.1)
Loans Charged-Off	(0.2)	0.0
Ending balance	4.1	0.6
Financing Receivable, Allowance for Credit Loss, Recovery	$ 1.1	$ 0.0