Loans Held for Investment - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 226.1	$ 247.2	$ 220.1	$ 122.3
Provision for (reduction of) credit losses	11.7	(1.7)	26.9	59.6
Loans Charged-Off(2)	(14.1)	(3.7)	(23.0)	(23.6)
Recoveries Collected	2.7	3.4	5.4	6.6
Ending balance	224.6	220.4	224.6	220.4
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	9.9	(1.7)	22.1	55.6
Provisional ACL Recorded for PCD loans		(24.8)		59.5
Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		45.2		17.3
Provision for (reduction of) credit losses		(8.8)		(2.1)
Loans Charged-Off(2)		0.0		(2.9)
Recoveries Collected		0.0		0.0
Ending balance		29.5		29.5
Provisional ACL Recorded for PCD loans		(6.9)		17.2
Owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		25.2		13.3
Provision for (reduction of) credit losses		1.4		6.0
Loans Charged-Off(2)		0.0		(2.2)
Recoveries Collected		0.0		0.0
Ending balance		26.6		26.6
Provisional ACL Recorded for PCD loans		0.0		9.5
Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		54.6		13.3
Provision for (reduction of) credit losses		(2.9)		8.5
Loans Charged-Off(2)		0.0		0.0
Recoveries Collected		0.7		0.7
Ending balance		33.4		33.4
Provisional ACL Recorded for PCD loans		(19.0)		10.9
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		125.0		43.9
Provision for (reduction of) credit losses		(10.3)		12.4
Loans Charged-Off(2)		0.0	(4.6)	(5.1)
Recoveries Collected		0.7		0.7
Ending balance		89.5		89.5
Provisional ACL Recorded for PCD loans		(25.9)		37.6
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.4	(1.0)	3.2	0.3
Provision for (reduction of) credit losses	0.2	6.3	(0.9)	5.8
Loans Charged-Off(2)	0.0	(0.1)	0.0	(5.4)
Recoveries Collected	0.1	0.2	0.4	1.3
Ending balance	2.7	6.5	2.7	6.5
Provisional ACL Recorded for PCD loans		1.1		4.5
Commercial and floor plans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		52.9		27.1
Provision for (reduction of) credit losses		1.9		20.3
Loans Charged-Off(2)		(1.1)		(5.3)
Recoveries Collected		0.4		0.8
Ending balance		54.1		54.1
Provisional ACL Recorded for PCD loans		0.0		11.2
Commercial purpose secured by 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		5.0		4.4
Provision for (reduction of) credit losses		0.4		0.8
Loans Charged-Off(2)		0.0		0.0
Recoveries Collected		0.1		0.1
Ending balance		5.5		5.5
Provisional ACL Recorded for PCD loans		0.0		0.2
Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		3.3		2.4
Provision for (reduction of) credit losses		0.1		1.0
Loans Charged-Off(2)		0.0		0.0
Recoveries Collected		0.0		0.0
Ending balance		3.4		3.4
Provisional ACL Recorded for PCD loans		0.0		0.0
Commercial construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		10.2		6.0
Provision for (reduction of) credit losses		4.7		8.7
Loans Charged-Off(2)		0.0		0.0
Recoveries Collected		0.0		0.0
Ending balance		14.9		14.9
Provisional ACL Recorded for PCD loans		0.0		0.2
Total construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		14.0		8.9
Provision for (reduction of) credit losses		5.0		9.1
Loans Charged-Off(2)		0.0		(2.7)
Recoveries Collected		0.1		0.2
Ending balance		19.1		19.1
Provisional ACL Recorded for PCD loans		0.0		3.6
Residential 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		18.7		13.4
Provision for (reduction of) credit losses		(0.7)		4.5
Loans Charged-Off(2)		0.0		(0.1)
Recoveries Collected		0.2		0.3
Ending balance		18.2		18.2
Provisional ACL Recorded for PCD loans		0.0		0.1
Home equity and HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		1.4		1.2
Provision for (reduction of) credit losses		(0.1)		0.0
Loans Charged-Off(2)		0.0		0.0
Recoveries Collected		0.2		0.3
Ending balance		1.5		1.5
Provisional ACL Recorded for PCD loans		0.0		0.0
Residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		20.1		14.6
Provision for (reduction of) credit losses		(0.8)		4.5
Loans Charged-Off(2)		0.0	(0.5)	(0.1)
Recoveries Collected		0.4		0.6
Ending balance		19.7		19.7
Provisional ACL Recorded for PCD loans		0.0		0.1
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		10.5		1.9
Provision for (reduction of) credit losses		(5.1)		1.3
Loans Charged-Off(2)		(0.1)	0.0	(0.2)
Recoveries Collected		0.1		0.1
Ending balance		5.4		5.4
Provisional ACL Recorded for PCD loans		0.0		2.3
Total real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	147.5	169.6	138.7	69.3
Provision for (reduction of) credit losses	20.4	(11.2)	33.8	27.3
Loans Charged-Off(2)	(10.0)	(0.1)	(14.8)	(8.1)
Recoveries Collected	0.8	1.3	1.0	1.6
Ending balance	158.7	133.7	158.7	133.7
Provisional ACL Recorded for PCD loans		(25.9)		43.6
Indirect consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		13.5		14.3
Provision for (reduction of) credit losses		(0.4)		(0.7)
Loans Charged-Off(2)		(0.8)	(3.4)	(1.7)
Recoveries Collected		0.8		1.2
Ending balance		13.1		13.1
Provisional ACL Recorded for PCD loans		0.0		0.0
Direct and advance lines
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		4.6		4.6
Provision for (reduction of) credit losses		0.5		0.4
Loans Charged-Off(2)		(0.6)	(1.9)	(1.4)
Recoveries Collected		0.5		1.4
Ending balance		5.0		5.0
Provisional ACL Recorded for PCD loans		0.0		0.0
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		2.3		2.2
Provision for (reduction of) credit losses		0.7		1.3
Loans Charged-Off(2)		(0.9)	(1.4)	(1.5)
Recoveries Collected		0.1		0.2
Ending balance		2.2		2.2
Provisional ACL Recorded for PCD loans		0.0		0.0
Total consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	21.8	20.4	23.3	21.1
Provision for (reduction of) credit losses	(6.2)	0.8	(5.5)	1.0
Loans Charged-Off(2)	(3.3)	(2.3)	(6.7)	(4.6)
Recoveries Collected	1.4	1.4	2.6	2.8
Ending balance	13.7	20.3	13.7	20.3
Provisional ACL Recorded for PCD loans		0.0		0.0
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		0.3		0.1
Provision for (reduction of) credit losses		0.1		0.4
Loans Charged-Off(2)		(0.1)		(0.2)
Recoveries Collected		0.0		0.0
Ending balance		0.3		0.3
Provisional ACL Recorded for PCD loans		0.0		0.0
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	54.4	58.2	54.9	31.6
Provision for (reduction of) credit losses	(4.5)	2.4	(5.3)	21.5
Loans Charged-Off(2)	(0.8)	(1.2)	(1.5)	(5.5)
Recoveries Collected	0.4	0.5	1.4	0.9
Ending balance	$ 49.5	59.9	$ 49.5	59.9
Provisional ACL Recorded for PCD loans		0.0		11.4
Land Acquisition And Development Construction Financing Receivable [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		0.5		0.5
Provision for (reduction of) credit losses		0.2		(0.6)
Loans Charged-Off(2)		0.0		(2.7)
Recoveries Collected		0.1		0.2
Ending balance		0.8		0.8
Provisional ACL Recorded for PCD loans		$ 0.0		$ 3.4