Loans Held for Investment - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	$ 59.5
Provision charged (credited) to operating expense	82.7	$ (14.6)	$ 56.9
Less loans charged-off	(45.0)	(16.6)	(20.9)
Recoveries Collected	14.9	9.3	6.7
Accounts Receivable, Allowance for Credit Loss	220.1	122.3	144.3	$ 73.0
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	68.4	(14.7)	55.5
Great Western Bank
Financing Receivable, Allowance for Credit Losses
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	68.3
Non Owner Occupied Loans
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	17.2
Provision charged (credited) to operating expense	(4.2)	(8.3)	11.7
Less loans charged-off	(3.5)	0.0	0.0
Recoveries Collected	0.4	0.1	0.1
Accounts Receivable, Allowance for Credit Loss	27.2	17.3	25.5	8.8
Commercial Real Estate Owner Occupied
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	9.5
Provision charged (credited) to operating expense	(2.7)	(2.7)	5.0
Less loans charged-off	(2.5)	(2.3)	(0.4)
Recoveries Collected	1.9	0.0	0.2
Accounts Receivable, Allowance for Credit Loss	19.5	13.3	18.3	10.0
Commercial Real Estate Multifamily
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	10.9
Provision charged (credited) to operating expense	8.7	2.3	3.4
Less loans charged-off	(5.7)	0.0	0.0
Recoveries Collected	0.7	0.0	0.0
Accounts Receivable, Allowance for Credit Loss	27.9	13.3	11.0	0.7
Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	37.6
Provision charged (credited) to operating expense	1.8	(8.7)	20.1
Less loans charged-off	(11.7)	(2.3)	(0.4)
Recoveries Collected	3.0	0.1	0.3
Accounts Receivable, Allowance for Credit Loss	74.6	43.9	54.8	19.5
Land Acquisition And Development Construction Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	3.4
Provision charged (credited) to operating expense	(0.4)	(0.1)	(0.4)
Less loans charged-off	(2.6)	(1.2)	(0.5)
Recoveries Collected	0.4	0.5	0.4
Accounts Receivable, Allowance for Credit Loss	1.3	0.5	1.3	1.9
Residential Real Estate Construction Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	0.0
Provision charged (credited) to operating expense	1.1	0.9	1.0
Less loans charged-off	0.0	(0.1)	0.0
Recoveries Collected	0.1	0.0	0.0
Accounts Receivable, Allowance for Credit Loss	3.6	2.4	1.6	1.5
Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	0.2
Provision charged (credited) to operating expense	31.6	(1.3)	3.3
Less loans charged-off	(6.6)	(0.1)	0.0
Recoveries Collected	0.0	0.1	0.0
Accounts Receivable, Allowance for Credit Loss	31.2	6.0	7.3	2.7
Construction Loans
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	3.6
Provision charged (credited) to operating expense	32.3	(0.5)	3.9
Less loans charged-off	(9.2)	(1.4)	(0.5)
Recoveries Collected	0.5	0.6	0.4
Accounts Receivable, Allowance for Credit Loss	36.1	8.9	10.2	6.1
Residential Real Estate 1-4 Family
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	0.1
Provision charged (credited) to operating expense	6.9	2.0	(1.1)
Less loans charged-off	(0.2)	0.0	0.0
Recoveries Collected	0.3	0.0	0.1
Accounts Receivable, Allowance for Credit Loss	20.5	13.4	11.4	1.8
Home equity line of credit
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	0.0
Provision charged (credited) to operating expense	0.0
Less loans charged-off	(0.1)
Recoveries Collected	0.5
Accounts Receivable, Allowance for Credit Loss	1.6	1.2
Residential Real Estate
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	0.1
Provision charged (credited) to operating expense	6.9	1.6	(1.5)
Less loans charged-off	(0.3)	(0.1)	0.0
Recoveries Collected	0.8	0.3	0.4
Accounts Receivable, Allowance for Credit Loss	22.1	14.6	12.8	2.8
Agricultural Real Estate Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	2.3
Provision charged (credited) to operating expense	1.5	(0.1)	0.4
Less loans charged-off	(0.2)	(0.7)	0.0
Recoveries Collected	0.4	0.0	0.0
Accounts Receivable, Allowance for Credit Loss	5.9	1.9	2.7	0.5
Real Estate Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	43.6
Provision charged (credited) to operating expense	42.5	(7.7)	22.9
Less loans charged-off	(21.4)	(4.5)	(0.9)
Recoveries Collected	4.7	1.0	1.1
Accounts Receivable, Allowance for Credit Loss	138.7	69.3	80.5	28.9
Indirect consumer
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	0.0
Provision charged (credited) to operating expense	2.7	(1.4)	5.4
Less loans charged-off	(4.0)	(3.5)	(4.1)
Recoveries Collected	2.3	2.5	2.1
Accounts Receivable, Allowance for Credit Loss	15.3	14.3	16.7	4.5
Other consumer
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	0.0
Provision charged (credited) to operating expense	2.2	1.7	1.6
Less loans charged-off	(3.7)	(2.9)	(3.9)
Recoveries Collected	2.1	1.2	1.0
Accounts Receivable, Allowance for Credit Loss	5.2	4.6	4.6	2.9
Credit card consumer
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	0.0
Provision charged (credited) to operating expense	2.4	0.6	1.8
Less loans charged-off	(2.4)	(1.8)	(2.8)
Recoveries Collected	0.6	0.8	0.8
Accounts Receivable, Allowance for Credit Loss	2.8	2.2	2.6	2.5
Total consumer loans
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	0.0
Provision charged (credited) to operating expense	7.3	0.9	8.8
Less loans charged-off	(10.1)	(8.2)	(10.8)
Recoveries Collected	5.0	4.5	3.9
Accounts Receivable, Allowance for Credit Loss	23.3	21.1	23.9	9.9
Commercial and Floor Plans
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	11.2
Provision charged (credited) to operating expense	15.1	(7.3)	20.4
Less loans charged-off	(6.6)	(3.0)	(8.0)
Recoveries Collected	2.2	3.2	1.4
Accounts Receivable, Allowance for Credit Loss	49.0	27.1	34.2	25.5
Commercial Purpose Secured by 1-4 Family
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	0.2
Provision charged (credited) to operating expense	1.2	(0.5)	2.5
Less loans charged-off	(0.2)	(0.3)	(0.1)
Recoveries Collected	0.1	0.5	0.2
Accounts Receivable, Allowance for Credit Loss	5.7	4.4	4.7	5.9
Commercial Borrower
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	0.0
Provision charged (credited) to operating expense	1.4	0.1	1.1
Less loans charged-off	(1.3)	(0.4)	(1.0)
Recoveries Collected	0.0	0.1	0.1
Accounts Receivable, Allowance for Credit Loss	0.2	0.1	0.3	1.2
Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	11.4
Provision charged (credited) to operating expense	17.7	(7.7)	24.0
Less loans charged-off	(8.1)	(3.7)	(9.1)
Recoveries Collected	2.3	3.8	1.7
Accounts Receivable, Allowance for Credit Loss	54.9	31.6	39.2	32.6
Agricultural Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses
Financing Receivable, Purchased with Credit Deterioration, Allowance for Credit Loss at Acquisition Date	4.5
Provision charged (credited) to operating expense	0.9	(0.2)	(0.2)
Less loans charged-off	(5.4)	(0.2)	(0.1)
Recoveries Collected	2.9	0.0	0.0
Accounts Receivable, Allowance for Credit Loss	$ 3.2	0.3	0.7	1.6
Home Equity Loan
Financing Receivable, Allowance for Credit Losses
Provision charged (credited) to operating expense		(0.4)	(0.4)
Less loans charged-off		(0.1)	0.0
Recoveries Collected		0.3	0.3
Accounts Receivable, Allowance for Credit Loss		$ 1.2	$ 1.4	$ 1.0