Loans Held for Investment - Schedule of Allowance for Credit Losses for Loans Held for Investment (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($) Investments	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) Investments	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($) Investments
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 224.6	$ 220.4	$ 220.1	$ 122.3
Provision charged to operating expense	(0.1)	8.4	26.8	68.0
Loans Charged-Off(2)	(6.2)	(16.2)	(29.2)	(39.8)
Ending balance	226.7	213.0	226.7	213.0
Provisional ACL Recorded for PCD loans				59.5
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	3.2	4.6	25.3	60.2
Financing Receivable, Allowance for Credit Loss, Recovery	5.1	4.2	10.5	10.8
Interest Receivable	129.2		129.2		$ 118.3
Debt Securities, Available-for-sale, Realized Loss	$ 0.0	(46.3)	$ (23.5)	(46.3)
Investment securities in an unrealized loss position (securities) | Investments	1,034		1,034		1,222
Debt Securities, Available-for-sale, Realized Gain	$ 0.0	0.0		0.0
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	7,066.8		$ 7,066.8		$ 4,998.9
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	6,169.7		6,169.7		4,432.0
Callable Within One Year
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Amortized cost of investment securities callable after one year but within five years	1,592.1		1,592.1
Fair value of investment securities callable after one year but within five years	1,514.4		1,514.4
Callable structured notes amortized costs	0.0		0.0
Debt Securities
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Interest Receivable	36.5		36.5		$ 38.9
Commercial Real Estate Non Owner Occupied Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		29.5		17.3
Provision charged to operating expense		(5.5)		(7.6)
Loans Charged-Off(2)		(0.1)		(3.0)
Ending balance		23.9		23.9
Provisional ACL Recorded for PCD loans				17.2
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	2.7	6.5	3.2	0.3
Provision charged to operating expense	0.5	(2.5)	(0.4)	3.3
Loans Charged-Off(2)	0.0	0.0	0.0	(5.4)
Ending balance	3.2	4.0	3.2	4.0
Provisional ACL Recorded for PCD loans				4.5
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0	0.4	1.3
Commercial and Floor Plans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		54.1		27.1
Provision charged to operating expense		(2.2)		18.1
Loans Charged-Off(2)		(0.5)		(5.8)
Ending balance		52.4		52.4
Provisional ACL Recorded for PCD loans				11.2
Financing Receivable, Allowance for Credit Loss, Recovery		1.0		1.8
Commercial Real Estate Owner Occupied [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		26.6		13.3
Provision charged to operating expense		(3.7)		2.3
Loans Charged-Off(2)		0.0		(2.2)
Ending balance		24.7		24.7
Provisional ACL Recorded for PCD loans				9.5
Financing Receivable, Allowance for Credit Loss, Recovery		1.8		1.8
Commercial Real Estate Multi-family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		33.4		13.3
Provision charged to operating expense		(2.6)		5.9
Loans Charged-Off(2)		(5.7)		(5.7)
Ending balance		25.1		25.1
Provisional ACL Recorded for PCD loans				10.9
Financing Receivable, Allowance for Credit Loss, Recovery		0.0		0.7
Commercial real estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		89.5		43.9
Provision charged to operating expense		(11.8)		0.6
Loans Charged-Off(2)		(5.8)	(6.3)	(10.9)
Ending balance		73.7		73.7
Provisional ACL Recorded for PCD loans				37.6
Financing Receivable, Allowance for Credit Loss, Recovery		1.8		2.5
Land Acquisition And Development Construction Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		0.8		0.5
Provision charged to operating expense		(0.2)		(0.8)
Loans Charged-Off(2)		0.0		(2.7)
Ending balance		0.7		0.7
Provisional ACL Recorded for PCD loans				3.4
Financing Receivable, Allowance for Credit Loss, Recovery		0.1		0.3
Residential construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		3.4		2.4
Provision charged to operating expense		(0.3)		0.7
Loans Charged-Off(2)		0.0		0.0
Ending balance		3.1		3.1
Provisional ACL Recorded for PCD loans				0.0
Financing Receivable, Allowance for Credit Loss, Recovery		0.0		0.0
Commercial construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		14.9		6.0
Provision charged to operating expense		16.4		25.1
Loans Charged-Off(2)		(6.6)		(6.6)
Ending balance		24.7		24.7
Provisional ACL Recorded for PCD loans				0.2
Financing Receivable, Allowance for Credit Loss, Recovery		0.0		0.0
Construction Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		19.1		8.9
Provision charged to operating expense		15.9		25.0
Loans Charged-Off(2)		(6.6)		(9.3)
Ending balance		28.5		28.5
Provisional ACL Recorded for PCD loans				3.6
Financing Receivable, Allowance for Credit Loss, Recovery		0.1		0.3
Residential Real Estate 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		18.2		13.4
Provision charged to operating expense		0.9		5.4
Loans Charged-Off(2)		0.0		(0.1)
Ending balance		19.1		19.1
Provisional ACL Recorded for PCD loans				0.1
Financing Receivable, Allowance for Credit Loss, Recovery		0.0		0.3
Home Equity Line of Credit [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		1.5		1.2
Provision charged to operating expense		0.1		0.1
Loans Charged-Off(2)		(0.1)		(0.1)
Ending balance		1.6		1.6
Provisional ACL Recorded for PCD loans				0.0
Financing Receivable, Allowance for Credit Loss, Recovery		0.1		0.4
Residential Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		19.7		14.6
Provision charged to operating expense		1.0		5.5
Loans Charged-Off(2)		(0.1)	(0.6)	(0.2)
Ending balance		20.7		20.7
Provisional ACL Recorded for PCD loans				0.1
Financing Receivable, Allowance for Credit Loss, Recovery		0.1		0.7
Agricultural Real Estate Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		5.4		1.9
Provision charged to operating expense		0.4		1.7
Loans Charged-Off(2)		0.0	0.0	(0.2)
Ending balance		5.8		5.8
Provisional ACL Recorded for PCD loans				2.3
Financing Receivable, Allowance for Credit Loss, Recovery		0.0		0.1
Real Estate Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	158.7	133.7	138.7	69.3
Provision charged to operating expense	1.8	5.5	35.6	32.8
Loans Charged-Off(2)	(1.8)	(12.5)	(16.6)	(20.6)
Ending balance	162.2	128.7	162.2	128.7
Provisional ACL Recorded for PCD loans				43.6
Financing Receivable, Allowance for Credit Loss, Recovery	3.5	2.0	4.5	3.6
Consumer Indirect Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		13.1		14.3
Provision charged to operating expense		1.6		0.9
Loans Charged-Off(2)		(1.2)	(5.2)	(2.9)
Ending balance		14.1		14.1
Provisional ACL Recorded for PCD loans				0.0
Financing Receivable, Allowance for Credit Loss, Recovery		0.6		1.8
Direct consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		5.0		4.6
Provision charged to operating expense		1.3		1.7
Loans Charged-Off(2)		(1.4)	(3.2)	(2.8)
Ending balance		5.3		5.3
Provisional ACL Recorded for PCD loans				0.0
Financing Receivable, Allowance for Credit Loss, Recovery		0.4		1.8
Credit Card Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		2.2		2.2
Provision charged to operating expense		0.3		1.6
Loans Charged-Off(2)		(0.3)	(2.0)	(1.8)
Ending balance		2.4		2.4
Provisional ACL Recorded for PCD loans				0.0
Financing Receivable, Allowance for Credit Loss, Recovery		0.2		0.4
Consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13.7	20.3	23.3	21.1
Provision charged to operating expense	1.8	3.2	(3.7)	4.2
Loans Charged-Off(2)	(3.7)	(2.9)	(10.4)	(7.5)
Ending balance	12.9	21.8	12.9	21.8
Provisional ACL Recorded for PCD loans				0.0
Financing Receivable, Allowance for Credit Loss, Recovery	1.1	1.2	3.7	4.0
Commercial Purpose secured by 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		5.5		4.4
Provision charged to operating expense		0.4		1.2
Loans Charged-Off(2)		0.0		0.0
Ending balance		5.9		5.9
Provisional ACL Recorded for PCD loans				0.2
Financing Receivable, Allowance for Credit Loss, Recovery		0.0		0.1
Commercial Borrower [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		0.3		0.1
Provision charged to operating expense		0.2		0.6
Loans Charged-Off(2)		(0.3)		(0.5)
Ending balance		0.2		0.2
Provisional ACL Recorded for PCD loans				0.0
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	49.5	59.9	54.9	31.6
Provision charged to operating expense	(0.9)	(1.6)	(6.2)	19.9
Loans Charged-Off(2)	(0.7)	(0.8)	(2.2)	(6.3)
Ending balance	48.4	58.5	48.4	58.5
Provisional ACL Recorded for PCD loans				11.4
Financing Receivable, Allowance for Credit Loss, Recovery	$ 0.5	$ 1.0	$ 1.9	$ 1.9