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Investment Securities - Debt Securities Issued By Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Investment securities |  |
Fair value investments | $ 992.9 | $ 547.4
AAA |  |
Investment securities |  |
Fair value investments | 12.0 | 10.6
AA |  |
Investment securities |  |
Fair value investments | 85.0 | 57.9
A |  |
Investment securities |  |
Fair value investments | 490.4 | 318.3
BBB |  |
Investment securities |  |
Fair value investments | 396.8 | 159.6
BB |  |
Investment securities |  |
Fair value investments | 0.0 | 1.0
Other |  |
Investment securities |  |
Fair value investments | $ 8.7 | $ 0.0

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