Loans Held for Investment - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 220.4	$ 135.5	$ 122.3	$ 144.3
Provisional ACL Recorded for PCD loans	0.0		59.5
Provision for (reversal of) credit losses	8.4	0.0	68.0	(5.1)
Loans Charged-Off	(16.2)	(2.1)	(39.8)	(11.8)
Recoveries Collected	4.2	1.5	10.8	7.2
Ending balance	213.0	135.1	213.0	135.1
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	4.6	0.2	60.2	(4.6)
Great Western Bank
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	68.3
Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	29.5	22.9	17.3	25.5
Provisional ACL Recorded for PCD loans	0.0		17.2
Provision for (reversal of) credit losses	(5.5)	1.1	(7.6)	(1.6)
Loans Charged-Off	(0.1)	0.0	(3.0)	0.0
Recoveries Collected	0.0	0.0	0.0	0.1
Ending balance	23.9	24.0	23.9	24.0
Owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	26.6	16.6	13.3	18.3
Provisional ACL Recorded for PCD loans	0.0		9.5
Provision for (reversal of) credit losses	(3.7)	(0.6)	2.3	0.0
Loans Charged-Off	0.0	0.0	(2.2)	(2.3)
Recoveries Collected	1.8	0.0	1.8	0.0
Ending balance	24.7	16.0	24.7	16.0
Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	33.4	11.6	13.3	11.0
Provisional ACL Recorded for PCD loans	0.0		10.9
Provision for (reversal of) credit losses	(2.6)	0.2	5.9	0.8
Loans Charged-Off	(5.7)	0.0	(5.7)	0.0
Recoveries Collected	0.0	0.0	0.7	0.0
Ending balance	25.1	11.8	25.1	11.8
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	89.5	51.1	43.9	54.8
Provisional ACL Recorded for PCD loans	0.0		37.6
Provision for (reversal of) credit losses	(11.8)	0.7	0.6	(0.8)
Loans Charged-Off	(5.8)	0.0	(10.9)	(2.3)
Recoveries Collected	1.8	0.0	2.5	0.1
Ending balance	73.7	51.8	73.7	51.8
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	6.5	0.4	0.3	0.7
Provisional ACL Recorded for PCD loans	0.0		4.5
Provision for (reversal of) credit losses	(2.5)	0.0	3.3	(0.1)
Loans Charged-Off	0.0	0.0	(5.4)	(0.2)
Recoveries Collected	0.0	0.0	1.3	0.0
Ending balance	4.0	0.4	4.0	0.4
Commercial and floor plans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	54.1	29.0	27.1	34.2
Provisional ACL Recorded for PCD loans	0.0		11.2
Provision for (reversal of) credit losses	(2.2)	0.3	18.1	(4.9)
Loans Charged-Off	(0.5)	(0.1)	(5.8)	(2.2)
Recoveries Collected	1.0	0.2	1.8	2.3
Ending balance	52.4	29.4	52.4	29.4
Commercial purpose secured by 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.5	4.4	4.4	4.7
Provisional ACL Recorded for PCD loans	0.0		0.2
Provision for (reversal of) credit losses	0.4	(0.2)	1.2	(0.8)
Loans Charged-Off	0.0	0.0	0.0	(0.1)
Recoveries Collected	0.0	0.0	0.1	0.4
Ending balance	5.9	4.2	5.9	4.2
Land acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.8	1.0	0.5	1.3
Provisional ACL Recorded for PCD loans	0.0		3.4
Provision for (reversal of) credit losses	(0.2)	(0.2)	(0.8)	(0.8)
Loans Charged-Off	0.0	0.0	(2.7)	(0.1)
Recoveries Collected	0.1	0.1	0.3	0.5
Ending balance	0.7	0.9	0.7	0.9
Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3.4	1.5	2.4	1.6
Provisional ACL Recorded for PCD loans	0.0		0.0
Provision for (reversal of) credit losses	(0.3)	0.3	0.7	0.3
Loans Charged-Off	0.0	0.0	0.0	(0.1)
Recoveries Collected	0.0	0.0	0.0	0.0
Ending balance	3.1	1.8	3.1	1.8
Commercial construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	14.9	8.0	6.0	7.3
Provisional ACL Recorded for PCD loans	0.0		0.2
Provision for (reversal of) credit losses	16.4	(1.3)	25.1	(0.6)
Loans Charged-Off	(6.6)	0.0	(6.6)	(0.1)
Recoveries Collected	0.0	0.0	0.0	0.1
Ending balance	24.7	6.7	24.7	6.7
Total construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	19.1	10.5	8.9	10.2
Provisional ACL Recorded for PCD loans	0.0		3.6
Provision for (reversal of) credit losses	15.9	(1.2)	25.0	(1.1)
Loans Charged-Off	(6.6)	0.0	(9.3)	(0.3)
Recoveries Collected	0.1	0.1	0.3	0.6
Ending balance	28.5	9.4	28.5	9.4
Residential 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	18.2	13.4	13.4	11.4
Provisional ACL Recorded for PCD loans	0.0		0.1
Provision for (reversal of) credit losses	0.9	(0.2)	5.4	1.8
Loans Charged-Off	0.0	0.0	(0.1)	0.0
Recoveries Collected	0.0	0.0	0.3	0.0
Ending balance	19.1	13.2	19.1	13.2
Home equity and HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.5	1.4	1.2	1.4
Provisional ACL Recorded for PCD loans	0.0		0.0
Provision for (reversal of) credit losses	0.1	(0.1)	0.1	(0.2)
Loans Charged-Off	(0.1)	0.0	(0.1)	(0.1)
Recoveries Collected	0.1	0.1	0.4	0.3
Ending balance	1.6	1.4	1.6	1.4
Residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	19.7	14.8	14.6	12.8
Provisional ACL Recorded for PCD loans	0.0		0.1
Provision for (reversal of) credit losses	1.0	(0.3)	5.5	1.6
Loans Charged-Off	(0.1)	0.0	(0.2)	(0.1)
Recoveries Collected	0.1	0.1	0.7	0.3
Ending balance	20.7	14.6	20.7	14.6
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.4	3.0	1.9	2.7
Provisional ACL Recorded for PCD loans	0.0		2.3
Provision for (reversal of) credit losses	0.4	0.1	1.7	0.4
Loans Charged-Off	0.0	0.0	(0.2)	0.0
Recoveries Collected	0.0	0.0	0.1	0.0
Ending balance	5.8	3.1	5.8	3.1
Total real estate loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	133.7	79.4	69.3	80.5
Provisional ACL Recorded for PCD loans	0.0		43.6
Provision for (reversal of) credit losses	5.5	(0.7)	32.8	0.1
Loans Charged-Off	(12.5)	0.0	(20.6)	(2.7)
Recoveries Collected	2.0	0.2	3.6	1.0
Ending balance	128.7	78.9	128.7	78.9
Indirect consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13.1	15.8	14.3	16.7
Provisional ACL Recorded for PCD loans	0.0		0.0
Provision for (reversal of) credit losses	1.6	(0.1)	0.9	(0.3)
Loans Charged-Off	(1.2)	(0.8)	(2.9)	(2.9)
Recoveries Collected	0.6	0.5	1.8	1.9
Ending balance	14.1	15.4	14.1	15.4
Direct and advance lines
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.0	4.6	4.6	4.6
Provisional ACL Recorded for PCD loans	0.0		0.0
Provision for (reversal of) credit losses	1.3	0.5	1.7	1.2
Loans Charged-Off	(1.4)	(0.8)	(2.8)	(2.0)
Recoveries Collected	0.4	0.4	1.8	0.9
Ending balance	5.3	4.7	5.3	4.7
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.2	1.6	2.2	2.6
Provisional ACL Recorded for PCD loans	0.0		0.0
Provision for (reversal of) credit losses	0.3	0.4	1.6	0.0
Loans Charged-Off	(0.3)	(0.4)	(1.8)	(1.4)
Recoveries Collected	0.2	0.2	0.4	0.6
Ending balance	2.4	1.8	2.4	1.8
Total consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	20.3	22.0	21.1	23.9
Provisional ACL Recorded for PCD loans	0.0		0.0
Provision for (reversal of) credit losses	3.2	0.8	4.2	0.9
Loans Charged-Off	(2.9)	(2.0)	(7.5)	(6.3)
Recoveries Collected	1.2	1.1	4.0	3.4
Ending balance	21.8	21.9	21.8	21.9
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.3	0.3	0.1	0.3
Provisional ACL Recorded for PCD loans	0.0		0.0
Provision for (reversal of) credit losses	0.2	0.0	0.6	0.2
Loans Charged-Off	(0.3)	0.0	(0.5)	(0.3)
Recoveries Collected	0.0	0.0	0.0	0.1
Ending balance	0.2	0.3	0.2	0.3
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	59.9	33.7	31.6	39.2
Provisional ACL Recorded for PCD loans	0.0		11.4
Provision for (reversal of) credit losses	(1.6)	0.1	19.9	(5.5)
Loans Charged-Off	(0.8)	(0.1)	(6.3)	(2.6)
Recoveries Collected	1.0	0.2	1.9	2.8
Ending balance	$ 58.5	$ 33.9	$ 58.5	$ 33.9