Investment Securities - Narrative (Details)		6 Months Ended
	Feb. 01, 2022 USD ($)	Jun. 30, 2022 USD ($) Investments	Jun. 30, 2021 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($) Investments	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Transfer, Amount	$ 10,700,000
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
Debt Securities, Held-to-maturity, Allowance for Credit Loss		1,600,000	$ 0	$ 1,600,000	0	$ 0	$ 0
Interest Receivable		90,600,000			47,400,000
Debt Securities, Available-for-sale, Realized Gain		0	0
Debt Securities, Available-for-sale, Realized Loss		0	$ 0
Debt Securities, Available-for-sale, Amortized Cost		8,193,800,000			4,859,300,000
Securities Sold under Agreements to Repurchase, Fair Value of Collateral		4,871,300,000			2,617,800,000
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure		$ 4,526,500,000			$ 2,610,800,000
Investment securities in an unrealized loss position (securities) | Investments		1,155			285
Great Western Bank
Schedule of Held-to-maturity Securities [Line Items]
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
Schedule of Held-to-maturity Securities [Line Items]
Interest Receivable		$ 30,800,000			$ 16,600,000
Callable Within One Year
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost of investment securities callable after one year but within five years		813,900,000
Fair value of investment securities callable after one year but within five years		775,900,000
Callable structured notes amortized costs		$ 0