Loans Held for Investment - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 122.3	$ 144.3
Initial ACL Recorded for PCD loans	84.3
Provision for (reversal of) credit losses	61.3	(5.1)
Loans Charged-Off	(19.9)	(4.9)
Recoveries Collected	3.2	2.0
Ending balance	247.2	136.6
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	57.3	(4.8)
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	17.3	25.5
Initial ACL Recorded for PCD loans	24.1
Provision for (reversal of) credit losses	6.7	(2.5)
Loans Charged-Off	(2.9)	0.0
Recoveries Collected	0.0	0.0
Ending balance	45.2	23.0
Owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13.3	18.3
Initial ACL Recorded for PCD loans	9.5
Provision for (reversal of) credit losses	4.6	(0.8)
Loans Charged-Off	(2.2)	(0.1)
Recoveries Collected	0.0	0.0
Ending balance	25.2	17.4
Multi-family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13.3	11.0
Initial ACL Recorded for PCD loans	29.9
Provision for (reversal of) credit losses	11.4	0.8
Loans Charged-Off	0.0	0.0
Recoveries Collected	0.0	0.0
Ending balance	54.6	11.8
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	43.9	54.8
Initial ACL Recorded for PCD loans	63.5
Provision for (reversal of) credit losses	22.7	(2.5)
Loans Charged-Off	(5.1)	(0.1)
Recoveries Collected	0.0	0.0
Ending balance	125.0	52.2
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.3	0.7
Initial ACL Recorded for PCD loans	3.4
Provision for (reversal of) credit losses	(0.5)	(0.1)
Loans Charged-Off	(0.2)	0.0
Recoveries Collected	1.1	0.0
Ending balance	4.1	0.6
Commercial and floor plans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	27.1	34.2
Initial ACL Recorded for PCD loans	11.2
Provision for (reversal of) credit losses	18.4	(1.8)
Loans Charged-Off	(4.2)	(1.8)
Recoveries Collected	0.4	0.4
Ending balance	52.9	31.0
Commercial purpose secured by 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.4	4.7
Initial ACL Recorded for PCD loans	0.2
Provision for (reversal of) credit losses	0.4	(0.2)
Loans Charged-Off	0.0	0.0
Recoveries Collected	0.0	0.2
Ending balance	5.0	4.7
Land acquisition & development
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.5	1.3
Initial ACL Recorded for PCD loans	3.4
Provision for (reversal of) credit losses	(0.8)	(0.2)
Loans Charged-Off	(2.7)	0.0
Recoveries Collected	0.1	0.1
Ending balance	0.5	1.2
Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.4	1.6
Initial ACL Recorded for PCD loans	0.0
Provision for (reversal of) credit losses	0.9	(0.2)
Loans Charged-Off	0.0	0.0
Recoveries Collected	0.0	0.0
Ending balance	3.3	1.4
Commercial construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	6.0	7.3
Initial ACL Recorded for PCD loans	0.2
Provision for (reversal of) credit losses	4.0	(0.2)
Loans Charged-Off	0.0	0.0
Recoveries Collected	0.0	0.1
Ending balance	10.2	7.2
Total construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	8.9	10.2
Initial ACL Recorded for PCD loans	3.6
Provision for (reversal of) credit losses	4.1	(0.6)
Loans Charged-Off	(2.7)	0.0
Recoveries Collected	0.1	0.2
Ending balance	14.0	9.8
Residential 1-4 family
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13.4	11.4
Initial ACL Recorded for PCD loans	0.1
Provision for (reversal of) credit losses	5.2	0.1
Loans Charged-Off	(0.1)	0.0
Recoveries Collected	0.1	0.0
Ending balance	18.7	11.5
Home equity and HELOC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.2	1.4
Initial ACL Recorded for PCD loans	0.0
Provision for (reversal of) credit losses	0.1	0.0
Loans Charged-Off	0.0	(0.1)
Recoveries Collected	0.1	0.1
Ending balance	1.4	1.4
Residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	14.6	12.8
Initial ACL Recorded for PCD loans	0.1
Provision for (reversal of) credit losses	5.3	0.1
Loans Charged-Off	(0.1)	(0.1)
Recoveries Collected	0.2	0.1
Ending balance	20.1	12.9
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.9	2.7
Initial ACL Recorded for PCD loans	2.3
Provision for (reversal of) credit losses	6.4	0.1
Loans Charged-Off	(5.2)	0.0
Recoveries Collected	0.0	0.0
Ending balance	5.4	2.8
Total real estate loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	69.3	80.5
Initial ACL Recorded for PCD loans	69.5
Provision for (reversal of) credit losses	38.5	(2.9)
Loans Charged-Off	(13.1)	(0.2)
Recoveries Collected	0.3	0.3
Ending balance	164.5	77.7
Indirect consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	14.3	16.7
Initial ACL Recorded for PCD loans	0.0
Provision for (reversal of) credit losses	(0.3)	0.0
Loans Charged-Off	(0.9)	(1.3)
Recoveries Collected	0.4	0.6
Ending balance	13.5	16.0
Direct and advance lines
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.6	4.6
Initial ACL Recorded for PCD loans	0.0
Provision for (reversal of) credit losses	(0.1)	0.7
Loans Charged-Off	(0.8)	(0.8)
Recoveries Collected	0.9	0.3
Ending balance	4.6	4.8
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.2	2.6
Initial ACL Recorded for PCD loans	0.0
Provision for (reversal of) credit losses	0.6	(0.7)
Loans Charged-Off	(0.6)	(0.6)
Recoveries Collected	0.1	0.2
Ending balance	2.3	1.5
Total consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	21.1	23.9
Initial ACL Recorded for PCD loans	0.0
Provision for (reversal of) credit losses	0.2	0.0
Loans Charged-Off	(2.3)	(2.7)
Recoveries Collected	1.4	1.1
Ending balance	20.4	22.3
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.1	0.3
Initial ACL Recorded for PCD loans	0.0
Provision for (reversal of) credit losses	0.3	0.2
Loans Charged-Off	(0.1)	(0.2)
Recoveries Collected	0.0	0.0
Ending balance	0.3	0.3
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	31.6	39.2
Initial ACL Recorded for PCD loans	11.4
Provision for (reversal of) credit losses	19.1	(1.8)
Loans Charged-Off	(4.3)	(2.0)
Recoveries Collected	0.4	0.6
Ending balance	58.2	36.0
Agriculture Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.3	0.7
Initial ACL Recorded for PCD loans	3.4
Provision for (reversal of) credit losses	(0.5)	(0.1)
Loans Charged-Off	(0.2)	0.0
Recoveries Collected	1.1	0.0
Ending balance	$ 4.1	$ 0.6