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Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
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Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost | $ 40.1 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost | 1,004.4 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost | 1,373.9 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost | 3,889.4 |
Debt Securities, Available-for-Sale, Excluding Accrued Interest | 5,841.5 | $ 6,946.1
Amortized Cost | 6,307.8 | 7,573.5
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value | 39.4 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value | 938.7 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value | 1,255.4 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value | 3,608.0 |
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost | 309.1 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost | 346.6 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost | 654.0 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost | 1,899.0 |
Amortized Cost1 | 3,208.7 | 3,453.7
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity |  |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value | 307.5 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value | 329.4 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value | 579.1 |
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value | 1,658.0 |
Estimated Fair Value | 2,874.0 | 3,052.2
Available-for-sale Securities and Held-to-maturity Securities |  |
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value | 1,552.2 |
Securities sold under agreements to repurchase | 3,858.6 | 4,998.9
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure | $ 3,462.2 | $ 4,432.0

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