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Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Available-for-Sale, Amortized Cost |  |
Available-for-Sale Amortized Cost, Within One Year | $ 43.9 |
Available-for-Sale Amortized Cost, After One Year but Within Five Years | 1,188.4 |
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years | 1,216.0 |
Available-for-Sale Amortized Cost, After Ten Years | 3,817.7 |
Amortized Cost | 6,266.0 | $ 6,307.8
Available-for-Sale, Estimated Fair Value |  |
Available-for-Sale Estimated Fair Value, Within One Year | 43.1 |
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years | 1,104.4 |
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years | 1,103.8 |
Available-for-Sale Estimated Fair Value, After Ten Years | 3,522.0 |
Estimated Fair Value | 5,773.3 | 5,841.5
Held-to-Maturity, Amortized Cost |  |
Held-to-Maturity Amortized Cost, Within One Year | 3.0 |
Held-to-Maturity Amortized Cost, After One Year but Within Five Years | 369.5 |
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years | 624.8 |
Held-to-Maturity Amortized Cost, After Ten Years | 1,856.2 |
Amortized Cost1 | 2,853.5 | 3,208.7
Held-to-Maturity, Estimated Fair Value |  |
Held-to-Maturity Estimated Fair Value, Within One Year | 3.0 |
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years | 349.9 |
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years | 546.2 |
Held-to-Maturity Estimated Fair Value, After Ten Years | 1,598.7 |
Investment debt securities held-to-maturity | 2,497.8 | 2,874.0
Securities Sold under Agreements to Repurchase [Abstract] |  |
Securities Sold under Agreements to Repurchase, Fair Value of Collateral | 3,427.6 | 3,858.6
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure | 3,009.9 | $ 3,462.2
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value | 1,577.2 |
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis | $ 1,632.5 |

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