Loans Held for Investment - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	$ (5.1)	$ 29.0
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	144.3	73.0
Loans Charged-Off	(4.9)	(4.6)
Recoveries Collected	2.0	1.5
Ending balance	136.6	129.1
Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	2.5	2.0
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	25.5	8.8
Loans Charged-Off	0.0	0.0
Recoveries Collected	0.0	0.0
Ending balance	23.0	15.7
Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.8	2.2
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	18.3	10.0
Loans Charged-Off	(0.1)	0.0
Recoveries Collected	0.0	0.0
Ending balance	17.4	15.7
Multi-family
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.8	0.5
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	11.0	0.7
Loans Charged-Off	0.0	0.0
Recoveries Collected	0.0	0.0
Ending balance	11.8	8.1
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	2.5	4.7
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	54.8	19.5
Loans Charged-Off	(0.1)	0.0
Recoveries Collected	0.0	0.0
Ending balance	52.2	39.5
Agricultural
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.1	0.1
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.7	1.6
Loans Charged-Off	0.0	0.0
Recoveries Collected	0.0	0.0
Ending balance	0.6	1.1
Commercial and floor plans
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	1.8	13.5
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	34.2	25.5
Loans Charged-Off	(1.8)	(0.8)
Recoveries Collected	0.4	0.4
Ending balance	31.0	33.5
Commercial purpose secured by 1-4 family
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.2	2.4
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.7	5.9
Loans Charged-Off	0.0	(0.1)
Recoveries Collected	0.2	0.0
Ending balance	4.7	4.4
Land acquisition & development
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.2	0.1
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.3	1.9
Loans Charged-Off	0.0	(0.5)
Recoveries Collected	0.1	0.0
Ending balance	1.2	1.4
Residential construction
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.2	0.5
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.6	1.5
Loans Charged-Off	0.0	0.0
Recoveries Collected	0.0	0.0
Ending balance	1.4	1.1
Commercial construction
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.2	2.1
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	7.3	2.7
Loans Charged-Off	0.0	0.0
Recoveries Collected	0.1	0.0
Ending balance	7.2	6.1
Total construction
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.6	2.7
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	10.2	6.1
Loans Charged-Off	0.0	(0.5)
Recoveries Collected	0.2	0.0
Ending balance	9.8	8.6
Residential 1-4 family
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.1	0.5
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	11.4	1.8
Loans Charged-Off	0.0	0.0
Recoveries Collected	0.0	0.0
Ending balance	11.5	12.9
Home equity and HELOC
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.0	0.1
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.4	1.0
Loans Charged-Off	(0.1)	0.0
Recoveries Collected	0.1	0.0
Ending balance	1.4	1.4
Residential
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.1	0.4
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	12.8	2.8
Loans Charged-Off	(0.1)	0.0
Recoveries Collected	0.1	0.0
Ending balance	12.9	14.3
Agricultural
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.1	0.3
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.7	0.5
Loans Charged-Off	0.0	0.0
Recoveries Collected	0.0	0.0
Ending balance	2.8	2.6
Total real estate loans
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	2.9	8.1
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	80.5	28.9
Loans Charged-Off	(0.2)	(0.5)
Recoveries Collected	0.3	0.0
Ending balance	77.7	65.0
Indirect consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.0	3.9
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	16.7	4.5
Loans Charged-Off	(1.3)	(1.2)
Recoveries Collected	0.6	0.7
Ending balance	16.0	16.7
Direct and advance lines
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.7	0.4
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.6	2.9
Loans Charged-Off	(0.8)	(1.0)
Recoveries Collected	0.3	0.2
Ending balance	4.8	5.5
Credit card
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.7	0.4
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.6	2.5
Loans Charged-Off	(0.6)	(0.8)
Recoveries Collected	0.2	0.2
Ending balance	1.5	2.6
Total consumer loans
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.0	4.7
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	23.9	9.9
Loans Charged-Off	(2.7)	(3.0)
Recoveries Collected	1.1	1.1
Ending balance	22.3	24.8
Credit card
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.2	0.4
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.3	1.2
Loans Charged-Off	(0.2)	(0.2)
Recoveries Collected	0.0	0.0
Ending balance	0.3	0.3
Commercial
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	1.8	16.3
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	39.2	32.6
Loans Charged-Off	(2.0)	(1.1)
Recoveries Collected	0.6	0.4
Ending balance	36.0	38.2
Agriculture Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.1	0.1
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.7	1.6
Loans Charged-Off	0.0	0.0
Recoveries Collected	0.0	0.0
Ending balance	$ 0.6	$ 1.1