Loans Held for Investment - Schedule of Allowance for Credit Losses for Loans Held for Investment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 135.5	$ 146.1	$ 144.3	$ 73.0
Provision charged to operating expense	0.0	(5.2)	5.1	(53.7)
Loans Charged-Off	(2.1)	(6.4)	(11.8)	(15.1)
Ending balance	135.1	145.5	135.1	145.5
Financing Receivable, Allowance for Credit Loss, Recovery	1.5	1.8	7.2	5.1
Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				30.0
Commercial Real Estate Non Owner Occupied Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	22.9	23.6	25.5	8.8
Provision charged to operating expense	(1.1)	(1.5)	(1.6)	(11.4)
Loans Charged-Off	0.0	0.0	0.0	0.0
Ending balance	24.0	25.2	24.0	25.2
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.1	0.1	0.1
Commercial Real Estate Non Owner Occupied Loans [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				4.9
Agricultural Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.4	0.9	0.7	1.6
Provision charged to operating expense	0.0	0.0	(0.1)	0.0
Loans Charged-Off	0.0	0.0	(0.2)	(0.1)
Ending balance	0.4	0.9	0.4	0.9
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0	0.0	0.0
Agricultural Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(0.6)
Commercial and Floor Plans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	29.0	41.0	34.2	25.5
Provision charged to operating expense	(0.3)	(1.2)	(4.9)	(20.0)
Loans Charged-Off	(0.1)	(3.3)	(2.2)	(4.7)
Ending balance	29.4	36.7	29.4	36.7
Financing Receivable, Allowance for Credit Loss, Recovery	0.2	0.2	2.3	1.0
Commercial and Floor Plans [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(5.1)
Commercial Real Estate Owner Occupied [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	16.6	19.1	18.3	10.0
Provision charged to operating expense	(0.6)	(0.1)	0.0	(5.7)
Loans Charged-Off	0.0	(0.2)	(2.3)	(0.3)
Ending balance	16.0	19.1	16.0	19.1
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.1	0.0	0.2
Commercial Real Estate Owner Occupied [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				3.5
Commercial Real Estate Multi-family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	11.6	8.5	11.0	0.7
Provision charged to operating expense	(0.2)	(2.3)	(0.8)	(3.2)
Loans Charged-Off	0.0	0.0	0.0	0.0
Ending balance	11.8	10.8	11.8	10.8
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0	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]
Beginning balance				6.9
Commercial real estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	51.1	51.2	54.8	19.5
Provision charged to operating expense	(0.7)	(3.9)	(0.8)	(20.3)
Loans Charged-Off	0.0	(0.2)	(2.3)	(0.3)
Ending balance	51.8	55.1	51.8	55.1
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.2	0.1	0.3
Commercial real estate | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				15.3
Land Acquisition And Development Construction Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.0	1.5	1.3	1.9
Provision charged to operating expense	(0.2)	(0.5)	(0.8)	(0.3)
Loans Charged-Off	0.0	0.0	(0.1)	(0.5)
Ending balance	0.9	1.2	0.9	1.2
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.2	0.5	0.2
Land Acquisition And Development Construction Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(0.1)
Residential construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.5	1.3	1.6	1.5
Provision charged to operating expense	(0.3)	(0.3)	(0.3)	(1.0)
Loans Charged-Off	0.0	0.0	(0.1)	0.0
Ending balance	1.8	1.6	1.8	1.6
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0	0.0	0.0
Residential construction | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(0.9)
Commercial construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	8.0	6.3	7.3	2.7
Provision charged to operating expense	(1.3)	(0.8)	(0.6)	(3.1)
Loans Charged-Off	0.0	0.0	(0.1)	0.0
Ending balance	6.7	7.1	6.7	7.1
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0	0.1	0.0
Commercial construction | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				1.3
Construction Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	10.5	9.1	10.2	6.1
Provision charged to operating expense	(1.2)	(0.6)	(1.1)	(3.8)
Loans Charged-Off	0.0	0.0	(0.3)	(0.5)
Ending balance	9.4	9.9	9.4	9.9
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.2	0.6	0.2
Construction Loans [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				0.3
Residential Real Estate 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13.4	10.3	11.4	1.8
Provision charged to operating expense	(0.2)	(1.1)	(1.8)	(3.3)
Loans Charged-Off	0.0	0.0	0.0	0.0
Ending balance	13.2	9.2	13.2	9.2
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0	0.0	0.1
Residential Real Estate 1-4 Family [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				10.6
Home Equity Line of Credit [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.4	1.5	1.4	1.0
Provision charged to operating expense	(0.1)	0.0	(0.2)	(0.1)
Loans Charged-Off	0.0	0.0	(0.1)	0.0
Ending balance	1.4	1.5	1.4	1.5
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.0	0.3	0.1
Residential Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	14.8	11.8	12.8	2.8
Provision charged to operating expense	(0.3)	(1.1)	(1.6)	(3.4)
Loans Charged-Off	0.0	0.0	(0.1)	0.0
Ending balance	14.6	10.7	14.6	10.7
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.0	0.3	0.2
Residential Real Estate [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				11.1
Agricultural Real Estate Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3.0	3.1	2.7	0.5
Provision charged to operating expense	(0.1)	(0.2)	(0.4)	(0.6)
Loans Charged-Off	0.0	0.0	0.0	0.0
Ending balance	3.1	2.9	3.1	2.9
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0	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]
Beginning balance				1.8
Real Estate Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	79.4	75.2	80.5	28.9
Provision charged to operating expense	(0.7)	(3.2)	(0.1)	(21.3)
Loans Charged-Off	0.0	(0.2)	(2.7)	(0.8)
Ending balance	78.9	78.6	78.9	78.6
Financing Receivable, Allowance for Credit Loss, Recovery	0.2	0.4	1.0	0.7
Real Estate Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				28.5
Consumer Indirect Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	15.8	16.4	16.7	4.5
Provision charged to operating expense	(0.1)	(0.7)	(0.3)	(5.1)
Loans Charged-Off	(0.8)	(0.8)	(2.9)	(3.2)
Ending balance	15.4	17.0	15.4	17.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.5	0.7	1.9	1.8
Consumer Indirect Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				8.8
Direct consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.6	5.1	4.6	2.9
Provision charged to operating expense	(0.5)	(0.6)	(1.2)	(1.3)
Loans Charged-Off	(0.8)	(1.1)	(2.0)	(3.1)
Ending balance	4.7	4.8	4.7	4.8
Financing Receivable, Allowance for Credit Loss, Recovery	0.4	0.2	0.9	0.7
Direct consumer | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				3.0
Credit Card Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.6	2.0	2.6	2.5
Provision charged to operating expense	(0.4)	(0.6)	0.0	(1.0)
Loans Charged-Off	(0.4)	(0.6)	(1.4)	(2.2)
Ending balance	1.8	2.2	1.8	2.2
Financing Receivable, Allowance for Credit Loss, Recovery	0.2	0.2	0.6	0.6
Credit Card Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				0.3
Consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	22.0	23.5	23.9	9.9
Provision charged to operating expense	(0.8)	(1.9)	(0.9)	(7.4)
Loans Charged-Off	(2.0)	(2.5)	(6.3)	(8.5)
Ending balance	21.9	24.0	21.9	24.0
Financing Receivable, Allowance for Credit Loss, Recovery	1.1	1.1	3.4	3.1
Consumer | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				12.1
Commercial Purpose secured by 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.4	5.1	4.7	5.9
Provision charged to operating expense	(0.2)	(0.2)	(0.8)	(2.8)
Loans Charged-Off	0.0	0.0	(0.1)	(0.1)
Ending balance	4.2	5.0	4.2	5.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.1	0.4	0.2
Commercial Purpose secured by 1-4 Family [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(3.8)
Commercial Borrower [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.3	0.4	0.3	1.2
Provision charged to operating expense	0.0	(0.3)	(0.2)	(1.0)
Loans Charged-Off	0.0	(0.4)	(0.3)	(0.9)
Ending balance	0.3	0.3	0.3	0.3
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0	0.1	0.1
Commercial Borrower [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(1.1)
Commercial Portfolio Segment [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	33.7	46.5	39.2	32.6
Provision charged to operating expense	(0.1)	(1.1)	(5.5)	(23.8)
Loans Charged-Off	(0.1)	(3.7)	(2.6)	(5.7)
Ending balance	33.9	42.0	33.9	42.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.2	0.3	2.8	1.3
Commercial Portfolio Segment [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(10.0)
Agriculture Portfolio Segment [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.4	0.9
Provision charged to operating expense	0.0	0.0		0.0
Loans Charged-Off	0.0	0.0		(0.1)
Ending balance	0.4	0.9	$ 0.4	0.9
Financing Receivable, Allowance for Credit Loss, Recovery	$ 0.0	$ 0.0		0.0
Agriculture Portfolio Segment [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				$ (0.6)