Loans Held for Investment - Schedule of Allowance for Credit Losses for Loans Held for Investment (Details)		3 Months Ended		9 Months Ended
	Feb. 01, 2022 USD ($)	Sep. 30, 2022 USD ($) Investments	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($) Investments	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($) Investments
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		$ 220,400,000	$ 135,500,000	$ 122,300,000	$ 144,300,000
Provision charged to operating expense		8,400,000	0	68,000,000.0	(5,100,000)
Loans Charged-Off		(16,200,000)	(2,100,000)	(39,800,000)	(11,800,000)
Ending balance		213,000,000.0	135,100,000	213,000,000.0	135,100,000
Provisional ACL Recorded for PCD loans		0		59,500,000
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)		4,600,000	200,000	60,200,000	(4,600,000)
Financing Receivable, Allowance for Credit Loss, Recovery		4,200,000	1,500,000	10,800,000	7,200,000
Interest Receivable		106,400,000		106,400,000		$ 47,400,000
Debt Securities, Available-for-sale, Realized Loss		46,300,000	0	46,400,000	0
Debt Securities, Held-To-Maturity, After Allowance For Credit Loss, Transfer, Amount	$ 10,900,000
Debt Securities, Available-For-Sale, Amortized Cost, after Allowance for Credit Loss, Transfer, Amount	485,900,000
Debt Securities, Available-For-Sale, after Allowance for Credit Loss, Transfer, Amount	463,600,000
Debt Securities, Held-to-maturity, Transfer to Trading, Unrealized Gain (Loss)	0
Allowance for Credit Losses		$ 0		$ 0		$ 0
Investment securities in an unrealized loss position (securities) | Investments		1,239		1,239		285
Debt Securities, Available-for-sale, Realized Gain		$ 0	0	$ 0	0
Securities Sold under Agreements to Repurchase, Fair Value of Collateral		4,836,700,000		4,836,700,000		$ 2,617,800,000
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure		4,265,100,000		4,265,100,000		2,610,800,000
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Transfer, Amount	10,700,000
Callable Within One Year
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Amortized cost of investment securities callable after one year but within five years		777,700,000		777,700,000
Fair value of investment securities callable after one year but within five years		706,200,000		706,200,000
Callable structured notes amortized costs		0		0
Great Western Bank
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)		68,300,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Debt Securities, Available-For-Sale	2,356,900,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Debt Securities, Held-To-Maturity	$ 342,100,000
Debt Securities
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Interest Receivable		34,300,000		34,300,000		$ 16,600,000
Commercial Real Estate Non Owner Occupied Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		29,500,000	22,900,000	17,300,000	25,500,000
Provision charged to operating expense		(5,500,000)	1,100,000	(7,600,000)	(1,600,000)
Loans Charged-Off		(100,000)	0	(3,000,000.0)	0
Ending balance		23,900,000	24,000,000.0	23,900,000	24,000,000.0
Provisional ACL Recorded for PCD loans		0		17,200,000
Financing Receivable, Allowance for Credit Loss, Recovery		0	0	0	100,000
Agricultural Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		6,500,000	400,000	300,000	700,000
Provision charged to operating expense		(2,500,000)	0	3,300,000	(100,000)
Loans Charged-Off		0	0	(5,400,000)	(200,000)
Ending balance		4,000,000.0	400,000	4,000,000.0	400,000
Provisional ACL Recorded for PCD loans		0		4,500,000
Financing Receivable, Allowance for Credit Loss, Recovery		0	0	1,300,000	0
Commercial and Floor Plans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		54,100,000	29,000,000.0	27,100,000	34,200,000
Provision charged to operating expense		(2,200,000)	300,000	18,100,000	(4,900,000)
Loans Charged-Off		(500,000)	(100,000)	(5,800,000)	(2,200,000)
Ending balance		52,400,000	29,400,000	52,400,000	29,400,000
Provisional ACL Recorded for PCD loans		0		11,200,000
Financing Receivable, Allowance for Credit Loss, Recovery		1,000,000.0	200,000	1,800,000	2,300,000
Commercial Real Estate Owner Occupied [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		26,600,000	16,600,000	13,300,000	18,300,000
Provision charged to operating expense		(3,700,000)	(600,000)	2,300,000	0
Loans Charged-Off		0	0	(2,200,000)	(2,300,000)
Ending balance		24,700,000	16,000,000.0	24,700,000	16,000,000.0
Provisional ACL Recorded for PCD loans		0		9,500,000
Financing Receivable, Allowance for Credit Loss, Recovery		1,800,000	0	1,800,000	0
Commercial Real Estate Multi-family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		33,400,000	11,600,000	13,300,000	11,000,000.0
Provision charged to operating expense		(2,600,000)	200,000	5,900,000	800,000
Loans Charged-Off		(5,700,000)	0	(5,700,000)	0
Ending balance		25,100,000	11,800,000	25,100,000	11,800,000
Provisional ACL Recorded for PCD loans		0		10,900,000
Financing Receivable, Allowance for Credit Loss, Recovery		0	0	700,000	0
Commercial real estate
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		89,500,000	51,100,000	43,900,000	54,800,000
Provision charged to operating expense		(11,800,000)	700,000	600,000	(800,000)
Loans Charged-Off		(5,800,000)	0	(10,900,000)	(2,300,000)
Ending balance		73,700,000	51,800,000	73,700,000	51,800,000
Provisional ACL Recorded for PCD loans		0		37,600,000
Financing Receivable, Allowance for Credit Loss, Recovery		1,800,000	0	2,500,000	100,000
Land Acquisition And Development Construction Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		800,000	1,000,000.0	500,000	1,300,000
Provision charged to operating expense		(200,000)	(200,000)	(800,000)	(800,000)
Loans Charged-Off		0	0	(2,700,000)	(100,000)
Ending balance		700,000	900,000	700,000	900,000
Provisional ACL Recorded for PCD loans		0		3,400,000
Financing Receivable, Allowance for Credit Loss, Recovery		100,000	100,000	300,000	500,000
Residential construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		3,400,000	1,500,000	2,400,000	1,600,000
Provision charged to operating expense		(300,000)	300,000	700,000	300,000
Loans Charged-Off		0	0	0	(100,000)
Ending balance		3,100,000	1,800,000	3,100,000	1,800,000
Provisional ACL Recorded for PCD loans		0		0
Financing Receivable, Allowance for Credit Loss, Recovery		0	0	0	0
Commercial construction
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		14,900,000	8,000,000.0	6,000,000.0	7,300,000
Provision charged to operating expense		16,400,000	(1,300,000)	25,100,000	(600,000)
Loans Charged-Off		(6,600,000)	0	(6,600,000)	(100,000)
Ending balance		24,700,000	6,700,000	24,700,000	6,700,000
Provisional ACL Recorded for PCD loans		0		200,000
Financing Receivable, Allowance for Credit Loss, Recovery		0	0	0	100,000
Construction Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		19,100,000	10,500,000	8,900,000	10,200,000
Provision charged to operating expense		15,900,000	(1,200,000)	25,000,000.0	(1,100,000)
Loans Charged-Off		(6,600,000)	0	(9,300,000)	(300,000)
Ending balance		28,500,000	9,400,000	28,500,000	9,400,000
Provisional ACL Recorded for PCD loans		0		3,600,000
Financing Receivable, Allowance for Credit Loss, Recovery		100,000	100,000	300,000	600,000
Residential Real Estate 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		18,200,000	13,400,000	13,400,000	11,400,000
Provision charged to operating expense		900,000	(200,000)	5,400,000	1,800,000
Loans Charged-Off		0	0	(100,000)	0
Ending balance		19,100,000	13,200,000	19,100,000	13,200,000
Provisional ACL Recorded for PCD loans		0		100,000
Financing Receivable, Allowance for Credit Loss, Recovery		0	0	300,000	0
Home Equity Line of Credit [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		1,500,000	1,400,000	1,200,000	1,400,000
Provision charged to operating expense		100,000	(100,000)	100,000	(200,000)
Loans Charged-Off		(100,000)	0	(100,000)	(100,000)
Ending balance		1,600,000	1,400,000	1,600,000	1,400,000
Provisional ACL Recorded for PCD loans		0		0
Financing Receivable, Allowance for Credit Loss, Recovery		100,000	100,000	400,000	300,000
Residential Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		19,700,000	14,800,000	14,600,000	12,800,000
Provision charged to operating expense		1,000,000.0	(300,000)	5,500,000	1,600,000
Loans Charged-Off		(100,000)	0	(200,000)	(100,000)
Ending balance		20,700,000	14,600,000	20,700,000	14,600,000
Provisional ACL Recorded for PCD loans		0		100,000
Financing Receivable, Allowance for Credit Loss, Recovery		100,000	100,000	700,000	300,000
Agricultural Real Estate Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		5,400,000	3,000,000.0	1,900,000	2,700,000
Provision charged to operating expense		400,000	100,000	1,700,000	400,000
Loans Charged-Off		0	0	(200,000)	0
Ending balance		5,800,000	3,100,000	5,800,000	3,100,000
Provisional ACL Recorded for PCD loans		0		2,300,000
Financing Receivable, Allowance for Credit Loss, Recovery		0	0	100,000	0
Real Estate Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		133,700,000	79,400,000	69,300,000	80,500,000
Provision charged to operating expense		5,500,000	(700,000)	32,800,000	100,000
Loans Charged-Off		(12,500,000)	0	(20,600,000)	(2,700,000)
Ending balance		128,700,000	78,900,000	128,700,000	78,900,000
Provisional ACL Recorded for PCD loans		0		43,600,000
Financing Receivable, Allowance for Credit Loss, Recovery		2,000,000.0	200,000	3,600,000	1,000,000.0
Consumer Indirect Financing Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		13,100,000	15,800,000	14,300,000	16,700,000
Provision charged to operating expense		1,600,000	(100,000)	900,000	(300,000)
Loans Charged-Off		(1,200,000)	(800,000)	(2,900,000)	(2,900,000)
Ending balance		14,100,000	15,400,000	14,100,000	15,400,000
Provisional ACL Recorded for PCD loans		0		0
Financing Receivable, Allowance for Credit Loss, Recovery		600,000	500,000	1,800,000	1,900,000
Direct consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		5,000,000.0	4,600,000	4,600,000	4,600,000
Provision charged to operating expense		1,300,000	500,000	1,700,000	1,200,000
Loans Charged-Off		(1,400,000)	(800,000)	(2,800,000)	(2,000,000.0)
Ending balance		5,300,000	4,700,000	5,300,000	4,700,000
Provisional ACL Recorded for PCD loans		0		0
Financing Receivable, Allowance for Credit Loss, Recovery		400,000	400,000	1,800,000	900,000
Credit Card Receivable [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		2,200,000	1,600,000	2,200,000	2,600,000
Provision charged to operating expense		300,000	400,000	1,600,000	0
Loans Charged-Off		(300,000)	(400,000)	(1,800,000)	(1,400,000)
Ending balance		2,400,000	1,800,000	2,400,000	1,800,000
Provisional ACL Recorded for PCD loans		0		0
Financing Receivable, Allowance for Credit Loss, Recovery		200,000	200,000	400,000	600,000
Consumer
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		20,300,000	22,000,000.0	21,100,000	23,900,000
Provision charged to operating expense		3,200,000	800,000	4,200,000	900,000
Loans Charged-Off		(2,900,000)	(2,000,000.0)	(7,500,000)	(6,300,000)
Ending balance		21,800,000	21,900,000	21,800,000	21,900,000
Provisional ACL Recorded for PCD loans		0		0
Financing Receivable, Allowance for Credit Loss, Recovery		1,200,000	1,100,000	4,000,000.0	3,400,000
Commercial Purpose secured by 1-4 Family [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		5,500,000	4,400,000	4,400,000	4,700,000
Provision charged to operating expense		400,000	(200,000)	1,200,000	(800,000)
Loans Charged-Off		0	0	0	(100,000)
Ending balance		5,900,000	4,200,000	5,900,000	4,200,000
Provisional ACL Recorded for PCD loans		0		200,000
Financing Receivable, Allowance for Credit Loss, Recovery		0	0	100,000	400,000
Commercial Borrower [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		300,000	300,000	100,000	300,000
Provision charged to operating expense		200,000	0	600,000	200,000
Loans Charged-Off		(300,000)	0	(500,000)	(300,000)
Ending balance		200,000	300,000	200,000	300,000
Provisional ACL Recorded for PCD loans		0		0
Financing Receivable, Allowance for Credit Loss, Recovery		0	0	0	100,000
Commercial Portfolio Segment [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		59,900,000	33,700,000	31,600,000	39,200,000
Provision charged to operating expense		(1,600,000)	100,000	19,900,000	(5,500,000)
Loans Charged-Off		(800,000)	(100,000)	(6,300,000)	(2,600,000)
Ending balance		58,500,000	33,900,000	58,500,000	33,900,000
Provisional ACL Recorded for PCD loans		0		11,400,000
Financing Receivable, Allowance for Credit Loss, Recovery		$ 1,000,000.0	$ 200,000	$ 1,900,000	$ 2,800,000