Loans Held for Investment - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 220.1	$ 122.3
Provisional ACL Recorded for PCD loans		84.3
Provision for credit losses	15.2	61.3
Loans Charged-Off(2)	(8.9)	(19.9)
Recoveries Collected	2.7	3.2
Ending balance	226.1	247.2
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	12.2	57.3
Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	27.2	17.3
Provisional ACL Recorded for PCD loans		24.1
Provision for credit losses	1.3	6.7
Loans Charged-Off(2)	(2.5)	(2.9)
Recoveries Collected	0.0	0.0
Ending balance	26.0	45.2
Owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	19.5	13.3
Provisional ACL Recorded for PCD loans		9.5
Provision for credit losses	(0.2)	4.6
Loans Charged-Off(2)	(1.8)	(2.2)
Recoveries Collected	0.1	0.0
Ending balance	17.6	25.2
Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	27.9	13.3
Provisional ACL Recorded for PCD loans		29.9
Provision for credit losses	7.1	11.4
Loans Charged-Off(2)	0.0	0.0
Recoveries Collected	0.0	0.0
Ending balance	35.0	54.6
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	74.6	43.9
Provisional ACL Recorded for PCD loans		63.5
Provision for credit losses	8.2	22.7
Loans Charged-Off(2)	(4.3)	(5.1)
Recoveries Collected	0.1	0.0
Ending balance	78.6	125.0
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3.2	0.3
Provisional ACL Recorded for PCD loans		3.4
Provision for credit losses	(1.1)	(0.5)
Loans Charged-Off(2)	0.0	(5.3)
Recoveries Collected	0.3	1.1
Ending balance	2.4	(1.0)
Commercial and floor plans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	49.0	27.1
Provisional ACL Recorded for PCD loans		11.2
Provision for credit losses	(0.8)	18.4
Loans Charged-Off(2)	(0.6)	(4.2)
Recoveries Collected	0.9	0.4
Ending balance	48.5	52.9
Commercial purpose secured by 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.7	4.4
Provisional ACL Recorded for PCD loans		0.2
Provision for credit losses	(0.2)	0.4
Loans Charged-Off(2)	0.0	0.0
Recoveries Collected	0.1	0.0
Ending balance	5.6	5.0
Land acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.3	0.5
Provisional ACL Recorded for PCD loans		3.4
Provision for credit losses	0.3	(0.8)
Loans Charged-Off(2)	0.0	(2.7)
Recoveries Collected	0.0	0.1
Ending balance	1.6	0.5
Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3.6	2.4
Provisional ACL Recorded for PCD loans		0.0
Provision for credit losses	0.1	0.9
Loans Charged-Off(2)	0.0	0.0
Recoveries Collected	0.0	0.0
Ending balance	3.7	3.3
Commercial construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	31.2	6.0
Provisional ACL Recorded for PCD loans		0.2
Provision for credit losses	5.9	4.0
Loans Charged-Off(2)	0.0	0.0
Recoveries Collected	0.0	0.0
Ending balance	37.1	10.2
Total construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	36.1	8.9
Provisional ACL Recorded for PCD loans		3.6
Provision for credit losses	6.3	4.1
Loans Charged-Off(2)	0.0	(2.7)
Recoveries Collected	0.0	0.1
Ending balance	42.4	14.0
Residential 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	20.5	13.4
Provisional ACL Recorded for PCD loans		0.1
Provision for credit losses	(0.2)	5.2
Loans Charged-Off(2)	(0.1)	(0.1)
Recoveries Collected	0.0	0.1
Ending balance	20.2	18.7
Home equity and HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.6	1.2
Provisional ACL Recorded for PCD loans		0.0
Provision for credit losses	0.3	0.1
Loans Charged-Off(2)	(0.4)	0.0
Recoveries Collected	0.0	0.1
Ending balance	1.5	1.4
Residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	22.1	14.6
Provisional ACL Recorded for PCD loans		0.1
Provision for credit losses	0.1	5.3
Loans Charged-Off(2)	(0.5)	(0.1)
Recoveries Collected	0.0	0.2
Ending balance	21.7	20.1
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.9	1.9
Provisional ACL Recorded for PCD loans		2.3
Provision for credit losses	(1.2)	6.4
Loans Charged-Off(2)	0.0	(0.1)
Recoveries Collected	0.1	0.0
Ending balance	4.8	10.5
Total real estate loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	138.7	69.3
Provisional ACL Recorded for PCD loans		69.5
Provision for credit losses	13.4	38.5
Loans Charged-Off(2)	(4.8)	(8.0)
Recoveries Collected	0.2	0.3
Ending balance	147.5	169.6
Indirect consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	15.3	14.3
Provisional ACL Recorded for PCD loans		0.0
Provision for credit losses	0.6	(0.3)
Loans Charged-Off(2)	(1.8)	(0.9)
Recoveries Collected	0.7	0.4
Ending balance	14.8	13.5
Direct and advance lines
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.2	4.6
Provisional ACL Recorded for PCD loans		0.0
Provision for credit losses	0.1	(0.1)
Loans Charged-Off(2)	(1.1)	(0.8)
Recoveries Collected	0.3	0.9
Ending balance	4.5	4.6
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.8	2.2
Provisional ACL Recorded for PCD loans		0.0
Provision for credit losses	0.0	0.6
Loans Charged-Off(2)	(0.5)	(0.6)
Recoveries Collected	0.2	0.1
Ending balance	2.5	2.3
Total consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	23.3	21.1
Provisional ACL Recorded for PCD loans		0.0
Provision for credit losses	0.7	0.2
Loans Charged-Off(2)	(3.4)	(2.3)
Recoveries Collected	1.2	1.4
Ending balance	21.8	20.4
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.2	0.1
Provisional ACL Recorded for PCD loans		0.0
Provision for credit losses	0.2	0.3
Loans Charged-Off(2)	(0.1)	(0.1)
Recoveries Collected	0.0	0.0
Ending balance	0.3	0.3
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	54.9	31.6
Provisional ACL Recorded for PCD loans		11.4
Provision for credit losses	(0.8)	19.1
Loans Charged-Off(2)	(0.7)	(4.3)
Recoveries Collected	1.0	0.4
Ending balance	54.4	58.2
Agriculture Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provisional ACL Recorded for PCD loans		$ 3.4
Consumer Home Equity and HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Loans Charged-Off(2)	$ (0.4)