Loans Held for Investment - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	$ 0.0	$ 19.5	$ (5.1)	$ 48.5
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	136.6	129.1	144.3	73.0
Loans Charged-Off	(4.8)	(4.1)	(9.7)	(8.7)
Recoveries Collected	3.7	1.8	5.7	3.3
Ending balance	135.5	146.1	135.5	146.1
Loans and Leases Receivable, Allowance			(4.8)
Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				30.0
Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.2	7.9	2.7	9.9
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	23.0	15.7	25.5	8.8
Loans Charged-Off	0.0	0.0	0.0	0.0
Recoveries Collected	0.1	0.0	0.1	0.0
Ending balance	22.9	23.6	22.9	23.6
Non-owner occupied | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				4.9
Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	1.4	3.4	0.6	5.6
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	17.4	15.7	18.3	10.0
Loans Charged-Off	(2.2)	(0.1)	(2.3)	(0.1)
Recoveries Collected	0.0	0.1	0.0	0.1
Ending balance	16.6	19.1	16.6	19.1
Owner occupied | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				3.5
Multi-family
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.2	0.4	0.6	0.9
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	11.8	8.1	11.0	0.7
Loans Charged-Off	0.0	0.0	0.0	0.0
Recoveries Collected	0.0	0.0	0.0	0.0
Ending balance	11.6	8.5	11.6	8.5
Multi-family | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				6.9
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	1.0	11.7	1.5	16.4
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	52.2	39.5	54.8	19.5
Loans Charged-Off	(2.2)	(0.1)	(2.3)	(0.1)
Recoveries Collected	0.1	0.1	0.1	0.1
Ending balance	51.1	51.2	51.1	51.2
Commercial real estate | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				15.3
Agricultural
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.0	0.1	0.1	0.0
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.6	1.1	0.7	1.6
Loans Charged-Off	(0.2)	(0.1)	(0.2)	(0.1)
Recoveries Collected	0.0	0.0	0.0	0.0
Ending balance	0.4	0.9	0.4	0.9
Agricultural | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(0.6)
Commercial and floor plans
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	3.4	7.7	5.2	21.2
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	31.0	33.5	34.2	25.5
Loans Charged-Off	(0.3)	(0.6)	(2.1)	(1.4)
Recoveries Collected	1.7	0.4	2.1	0.8
Ending balance	29.0	41.0	29.0	41.0
Commercial and floor plans | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(5.1)
Commercial purpose secured by 1-4 family
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.4	0.6	0.6	3.0
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.7	4.4	4.7	5.9
Loans Charged-Off	(0.1)	0.0	(0.1)	(0.1)
Recoveries Collected	0.2	0.1	0.4	0.1
Ending balance	4.4	5.1	4.4	5.1
Commercial purpose secured by 1-4 family | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(3.8)
Land acquisition & development
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.4	0.1	0.6	0.2
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.2	1.4	1.3	1.9
Loans Charged-Off	(0.1)	0.0	(0.1)	(0.5)
Recoveries Collected	0.3	0.0	0.4	0.0
Ending balance	1.0	1.5	1.0	1.5
Land acquisition & development | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(0.1)
Residential construction
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.2	0.2	0.0	0.7
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.4	1.1	1.6	1.5
Loans Charged-Off	(0.1)	0.0	(0.1)	0.0
Recoveries Collected	0.0	0.0	0.0	0.0
Ending balance	1.5	1.3	1.5	1.3
Residential construction | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(0.9)
Commercial construction
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.9	0.2	0.7	2.3
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	7.2	6.1	7.3	2.7
Loans Charged-Off	(0.1)	0.0	(0.1)	0.0
Recoveries Collected	0.0	0.0	0.1	0.0
Ending balance	8.0	6.3	8.0	6.3
Commercial construction | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				1.3
Total construction
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.7	0.5	0.1	3.2
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	9.8	8.6	10.2	6.1
Loans Charged-Off	(0.3)	0.0	(0.3)	(0.5)
Recoveries Collected	0.3	0.0	0.5	0.0
Ending balance	10.5	9.1	10.5	9.1
Total construction | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				0.3
Residential 1-4 family
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	1.9	2.7	2.0	2.2
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	11.5	12.9	11.4	1.8
Loans Charged-Off	0.0	0.0	0.0	0.0
Recoveries Collected	0.0	0.1	0.0	0.1
Ending balance	13.4	10.3	13.4	10.3
Residential 1-4 family | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				10.6
Home equity and HELOC
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.1	0.0	0.1	0.1
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.4	1.4	1.4	1.0
Loans Charged-Off	0.0	0.0	(0.1)	0.0
Recoveries Collected	0.1	0.1	0.2	0.1
Ending balance	1.4	1.5	1.4	1.5
Residential
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	1.8	2.7	1.9	2.3
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	12.9	14.3	12.8	2.8
Loans Charged-Off	0.0	0.0	(0.1)	0.0
Recoveries Collected	0.1	0.2	0.2	0.2
Ending balance	14.8	11.8	14.8	11.8
Residential | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				11.1
Agricultural
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.2	0.5	0.3	0.8
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.8	2.6	2.7	0.5
Loans Charged-Off	0.0	0.0	0.0	0.0
Recoveries Collected	0.0	0.0	0.0	0.0
Ending balance	3.0	3.1	3.0	3.1
Agricultural | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				1.8
Total real estate loans
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	3.7	10.0	0.8	18.1
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	77.7	65.0	80.5	28.9
Loans Charged-Off	(2.5)	(0.1)	(2.7)	(0.6)
Recoveries Collected	0.5	0.3	0.8	0.3
Ending balance	79.4	75.2	79.4	75.2
Total real estate loans | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				28.5
Indirect consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.2	0.5	0.2	4.4
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	16.0	16.7	16.7	4.5
Loans Charged-Off	(0.8)	(1.2)	(2.1)	(2.4)
Recoveries Collected	0.8	0.4	1.4	1.1
Ending balance	15.8	16.4	15.8	16.4
Indirect consumer | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				8.8
Direct and advance lines
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.0	0.3	0.7	0.7
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.8	5.5	4.6	2.9
Loans Charged-Off	(0.4)	(1.0)	(1.2)	(2.0)
Recoveries Collected	0.2	0.3	0.5	0.5
Ending balance	4.6	5.1	4.6	5.1
Direct and advance lines | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				3.0
Credit card
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.3	0.0	0.4	0.4
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.5	2.6	2.6	2.5
Loans Charged-Off	(0.4)	(0.8)	(1.0)	(1.6)
Recoveries Collected	0.2	0.2	0.4	0.4
Ending balance	1.6	2.0	1.6	2.0
Credit card | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				0.3
Total consumer loans
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.1	0.8	0.1	5.5
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	22.3	24.8	23.9	9.9
Loans Charged-Off	(1.6)	(3.0)	(4.3)	(6.0)
Recoveries Collected	1.2	0.9	2.3	2.0
Ending balance	22.0	23.5	22.0	23.5
Total consumer loans | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				12.1
Credit card
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.0	0.3	0.2	0.7
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.3	0.3	0.3	1.2
Loans Charged-Off	(0.1)	(0.3)	(0.3)	(0.5)
Recoveries Collected	0.1	0.1	0.1	0.1
Ending balance	0.3	0.4	0.3	0.4
Credit card | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(1.1)
Commercial
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	3.8	8.6	5.6	24.9
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	36.0	38.2	39.2	32.6
Loans Charged-Off	(0.5)	(0.9)	(2.5)	(2.0)
Recoveries Collected	2.0	0.6	2.6	1.0
Ending balance	33.7	46.5	33.7	46.5
Commercial | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(10.0)
Agriculture Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reversal of) provision for credit losses	0.0	0.1		0.0
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.6	1.1
Loans Charged-Off	(0.2)	(0.1)		(0.1)
Recoveries Collected	0.0	0.0		0.0
Ending balance	$ 0.4	$ 0.9	$ 0.4	0.9
Agriculture Portfolio Segment [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(0.6)
Consumer Home Equity and HELOC | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				$ 0.5