Loans Held for Investment - Schedule of Allowance for Credit Losses for Loans Held for Investment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 144.3	$ 73.0
Provision for Loan and Lease Losses	(5.1)	29.0
Loans Charged-Off	(4.9)	(4.6)
Ending balance	136.6	129.1
Financing Receivable, Allowance for Credit Loss, Recovery	2.0	1.5
Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		30.0
Commercial Real Estate Non Owner Occupied Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	25.5	8.8
Provision for Loan and Lease Losses	2.5	2.0
Loans Charged-Off	0.0	0.0
Ending balance	23.0	15.7
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Commercial Real Estate Non Owner Occupied Loans [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		4.9
Agricultural Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.7	1.6
Provision for Loan and Lease Losses	0.1	0.1
Loans Charged-Off	0.0	0.0
Ending balance	0.6	1.1
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Agricultural Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		0.6
Commercial and Floor Plans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	34.2	25.5
Provision for Loan and Lease Losses	1.8	13.5
Loans Charged-Off	(1.8)	(0.8)
Ending balance	31.0	33.5
Financing Receivable, Allowance for Credit Loss, Recovery	0.4	0.4
Commercial and Floor Plans [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		5.1
Commercial Real Estate Owner Occupied [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	18.3	10.0
Provision for Loan and Lease Losses	0.8	2.2
Loans Charged-Off	(0.1)	0.0
Ending balance	17.4	15.7
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Commercial Real Estate Owner Occupied [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		3.5
Commercial Real Estate Multi-family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	11.0	0.7
Provision for Loan and Lease Losses	0.8	0.5
Loans Charged-Off	0.0	0.0
Ending balance	11.8	8.1
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Commercial Real Estate Multi-family [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		6.9
Commercial real estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	54.8	19.5
Provision for Loan and Lease Losses	2.5	4.7
Loans Charged-Off	(0.1)	0.0
Ending balance	52.2	39.5
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Commercial real estate | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		15.3
Land Acquisition And Development Construction Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.3	1.9
Provision for Loan and Lease Losses	0.2	0.1
Loans Charged-Off	0.0	(0.5)
Ending balance	1.2	1.4
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.0
Land Acquisition And Development Construction Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		0.1
Residential construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.6	1.5
Provision for Loan and Lease Losses	0.2	0.5
Loans Charged-Off	0.0	0.0
Ending balance	1.4	1.1
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Residential construction | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		0.9
Commercial construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	7.3	2.7
Provision for Loan and Lease Losses	0.2	2.1
Loans Charged-Off	0.0	0.0
Ending balance	7.2	6.1
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.0
Commercial construction | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		1.3
Construction Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	10.2	6.1
Provision for Loan and Lease Losses	0.6	2.7
Loans Charged-Off	0.0	(0.5)
Ending balance	9.8	8.6
Financing Receivable, Allowance for Credit Loss, Recovery	0.2	0.0
Construction Loans [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		0.3
Residential Real Estate 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	11.4	1.8
Provision for Loan and Lease Losses	0.1	0.5
Loans Charged-Off	0.0	0.0
Ending balance	11.5	12.9
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Residential Real Estate 1-4 Family [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		10.6
Home Equity Line of Credit [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.4	1.0
Provision for Loan and Lease Losses	0.0	0.1
Loans Charged-Off	(0.1)	0.0
Ending balance	1.4	1.4
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.0
Home Equity Line of Credit [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		0.5
Residential Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	12.8	2.8
Provision for Loan and Lease Losses	0.1	0.4
Loans Charged-Off	(0.1)	0.0
Ending balance	12.9	14.3
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.0
Residential Real Estate [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		11.1
Agricultural Real Estate Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.7	0.5
Provision for Loan and Lease Losses	0.1	0.3
Loans Charged-Off	0.0	0.0
Ending balance	2.8	2.6
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Agricultural Real Estate Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		1.8
Real Estate Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	80.5	28.9
Provision for Loan and Lease Losses	2.9	8.1
Loans Charged-Off	(0.2)	(0.5)
Ending balance	77.7	65.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.3	0.0
Real Estate Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		28.5
Consumer Indirect Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	16.7	4.5
Provision for Loan and Lease Losses	0.0	3.9
Loans Charged-Off	(1.3)	(1.2)
Ending balance	16.0	16.7
Financing Receivable, Allowance for Credit Loss, Recovery	0.6	0.7
Consumer Indirect Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		8.8
Direct consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.6	2.9
Provision for Loan and Lease Losses	0.7	0.4
Loans Charged-Off	(0.8)	(1.0)
Ending balance	4.8	5.5
Financing Receivable, Allowance for Credit Loss, Recovery	0.3	0.2
Direct consumer | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		3.0
Credit Card Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.6	2.5
Provision for Loan and Lease Losses	0.7	0.4
Loans Charged-Off	(0.6)	(0.8)
Ending balance	1.5	2.6
Financing Receivable, Allowance for Credit Loss, Recovery	0.2	0.2
Credit Card Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		0.3
Consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	23.9	9.9
Provision for Loan and Lease Losses	0.0	4.7
Loans Charged-Off	(2.7)	(3.0)
Ending balance	22.3	24.8
Financing Receivable, Allowance for Credit Loss, Recovery	1.1	1.1
Consumer | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		12.1
Commercial Purpose secured by 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.7	5.9
Provision for Loan and Lease Losses	0.2	2.4
Loans Charged-Off	0.0	(0.1)
Ending balance	4.7	4.4
Financing Receivable, Allowance for Credit Loss, Recovery	0.2	0.0
Commercial Purpose secured by 1-4 Family [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		3.8
Commercial Borrower [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.3	1.2
Provision for Loan and Lease Losses	0.2	0.4
Loans Charged-Off	(0.2)	(0.2)
Ending balance	0.3	0.3
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Commercial Borrower [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		1.1
Commercial Portfolio Segment [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	39.2	32.6
Provision for Loan and Lease Losses	1.8	16.3
Loans Charged-Off	(2.0)	(1.1)
Ending balance	36.0	38.2
Financing Receivable, Allowance for Credit Loss, Recovery	0.6	0.4
Commercial Portfolio Segment [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		10.0
Agriculture Portfolio Segment [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.7	1.6
Provision for Loan and Lease Losses	0.1	0.1
Loans Charged-Off	0.0	0.0
Ending balance	0.6	1.1
Financing Receivable, Allowance for Credit Loss, Recovery	$ 0.0	0.0
Agriculture Portfolio Segment [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance		$ 0.6