Investment Securities - Narrative (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 ($)	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($) Investments	Jun. 30, 2021 USD ($)	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			$ 0
Debt Securities, Held-to-maturity, Allowance for Credit Loss		1,900,000	$ 0	1,900,000	$ 0	$ 1,600,000	0	$ 0	$ 0
Interest Receivable		106,400,000		106,400,000			47,400,000
Debt Securities, Available-for-sale, Realized Gain		0	0	0	0
Debt Securities, Available-for-sale, Realized Loss		46,300,000	$ 0	46,400,000	$ 0
Debt Securities, Available-for-sale, Amortized Cost		7,448,200,000		7,448,200,000			4,859,300,000
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
Investment securities in an unrealized loss position (securities) | Investments		1,239		1,239			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
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		$ 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