Loans Held for Investment - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 224.6	$ 220.4	$ 220.1	$ 122.3
(Reduction of) provision for credit losses	(0.1)	8.4	26.8	68.0
Loans Charged-Off(2)	(6.2)	(16.2)	(29.2)	(39.8)
Recoveries Collected	5.1	4.2	10.5	10.8
Ending balance	226.7	213.0	226.7	213.0
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	3.2	4.6	25.3	60.2
Provisional ACL Recorded for PCD loans				59.5
Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		29.5		17.3
(Reduction of) provision for credit losses		(5.5)		(7.6)
Loans Charged-Off(2)		(0.1)		(3.0)
Recoveries Collected		0.0		0.0
Ending balance		23.9		23.9
Provisional ACL Recorded for PCD loans				17.2
Owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		26.6		13.3
(Reduction of) provision for credit losses		(3.7)		2.3
Loans Charged-Off(2)		0.0		(2.2)
Recoveries Collected		1.8		1.8
Ending balance		24.7		24.7
Provisional ACL Recorded for PCD loans				9.5
Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		33.4		13.3
(Reduction of) provision for credit losses		(2.6)		5.9
Loans Charged-Off(2)		(5.7)		(5.7)
Recoveries Collected		0.0		0.7
Ending balance		25.1		25.1
Provisional ACL Recorded for PCD loans				10.9
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		89.5		43.9
(Reduction of) provision for credit losses		(11.8)		0.6
Loans Charged-Off(2)		(5.8)	(6.3)	(10.9)
Recoveries Collected		1.8		2.5
Ending balance		73.7		73.7
Provisional ACL Recorded for PCD loans				37.6
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.7	6.5	3.2	0.3
(Reduction of) provision for credit losses	0.5	(2.5)	(0.4)	3.3
Loans Charged-Off(2)	0.0	0.0	0.0	(5.4)
Recoveries Collected	0.0	0.0	0.4	1.3
Ending balance	3.2	4.0	3.2	4.0
Provisional ACL Recorded for PCD loans				4.5
Commercial and floor plans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		54.1		27.1
(Reduction of) provision for credit losses		(2.2)		18.1
Loans Charged-Off(2)		(0.5)		(5.8)
Recoveries Collected		1.0		1.8
Ending balance		52.4		52.4
Provisional ACL Recorded for PCD loans				11.2
Commercial purpose secured by 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		5.5		4.4
(Reduction of) provision for credit losses		0.4		1.2
Loans Charged-Off(2)		0.0		0.0
Recoveries Collected		0.0		0.1
Ending balance		5.9		5.9
Provisional ACL Recorded for PCD loans				0.2
Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		3.4		2.4
(Reduction of) provision for credit losses		(0.3)		0.7
Loans Charged-Off(2)		0.0		0.0
Recoveries Collected		0.0		0.0
Ending balance		3.1		3.1
Provisional ACL Recorded for PCD loans				0.0
Commercial construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		14.9		6.0
(Reduction of) provision for credit losses		16.4		25.1
Loans Charged-Off(2)		(6.6)		(6.6)
Recoveries Collected		0.0		0.0
Ending balance		24.7		24.7
Provisional ACL Recorded for PCD loans				0.2
Total construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		19.1		8.9
(Reduction of) provision for credit losses		15.9		25.0
Loans Charged-Off(2)		(6.6)		(9.3)
Recoveries Collected		0.1		0.3
Ending balance		28.5		28.5
Provisional ACL Recorded for PCD loans				3.6
Residential 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		18.2		13.4
(Reduction of) provision for credit losses		0.9		5.4
Loans Charged-Off(2)		0.0		(0.1)
Recoveries Collected		0.0		0.3
Ending balance		19.1		19.1
Provisional ACL Recorded for PCD loans				0.1
Home equity and HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		1.5		1.2
(Reduction of) provision for credit losses		0.1		0.1
Loans Charged-Off(2)		(0.1)		(0.1)
Recoveries Collected		0.1		0.4
Ending balance		1.6		1.6
Provisional ACL Recorded for PCD loans				0.0
Residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		19.7		14.6
(Reduction of) provision for credit losses		1.0		5.5
Loans Charged-Off(2)		(0.1)	(0.6)	(0.2)
Recoveries Collected		0.1		0.7
Ending balance		20.7		20.7
Provisional ACL Recorded for PCD loans				0.1
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		5.4		1.9
(Reduction of) provision for credit losses		0.4		1.7
Loans Charged-Off(2)		0.0	0.0	(0.2)
Recoveries Collected		0.0		0.1
Ending balance		5.8		5.8
Provisional ACL Recorded for PCD loans				2.3
Total real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	158.7	133.7	138.7	69.3
(Reduction of) provision for credit losses	1.8	5.5	35.6	32.8
Loans Charged-Off(2)	(1.8)	(12.5)	(16.6)	(20.6)
Recoveries Collected	3.5	2.0	4.5	3.6
Ending balance	162.2	128.7	162.2	128.7
Provisional ACL Recorded for PCD loans				43.6
Indirect consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		13.1		14.3
(Reduction of) provision for credit losses		1.6		0.9
Loans Charged-Off(2)		(1.2)	(5.2)	(2.9)
Recoveries Collected		0.6		1.8
Ending balance		14.1		14.1
Provisional ACL Recorded for PCD loans				0.0
Direct and advance lines
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		5.0		4.6
(Reduction of) provision for credit losses		1.3		1.7
Loans Charged-Off(2)		(1.4)	(3.2)	(2.8)
Recoveries Collected		0.4		1.8
Ending balance		5.3		5.3
Provisional ACL Recorded for PCD loans				0.0
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		2.2		2.2
(Reduction of) provision for credit losses		0.3		1.6
Loans Charged-Off(2)		(0.3)	(2.0)	(1.8)
Recoveries Collected		0.2		0.4
Ending balance		2.4		2.4
Provisional ACL Recorded for PCD loans				0.0
Total consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13.7	20.3	23.3	21.1
(Reduction of) provision for credit losses	1.8	3.2	(3.7)	4.2
Loans Charged-Off(2)	(3.7)	(2.9)	(10.4)	(7.5)
Recoveries Collected	1.1	1.2	3.7	4.0
Ending balance	12.9	21.8	12.9	21.8
Provisional ACL Recorded for PCD loans				0.0
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		0.3		0.1
(Reduction of) provision for credit losses		0.2		0.6
Loans Charged-Off(2)		(0.3)		(0.5)
Recoveries Collected		0.0		0.0
Ending balance		0.2		0.2
Provisional ACL Recorded for PCD loans				0.0
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	49.5	59.9	54.9	31.6
(Reduction of) provision for credit losses	(0.9)	(1.6)	(6.2)	19.9
Loans Charged-Off(2)	(0.7)	(0.8)	(2.2)	(6.3)
Recoveries Collected	0.5	1.0	1.9	1.9
Ending balance	$ 48.4	58.5	$ 48.4	58.5
Provisional ACL Recorded for PCD loans				11.4
Land Acquisition And Development Construction Financing Receivable [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		0.8		0.5
(Reduction of) provision for credit losses		(0.2)		(0.8)
Loans Charged-Off(2)		0.0		(2.7)
Recoveries Collected		0.1		0.3
Ending balance		$ 0.7		0.7
Provisional ACL Recorded for PCD loans				$ 3.4