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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 |  |  |  |  |

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