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Investment Securities - Debt securities issued by corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions | Mar. 31, 2022 | Dec. 31, 2021
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Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | $ 969.6 | $ 992.9
AAA |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 10.8 | 12.0
AA |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 84.3 | 85.0
A |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 468.3 | 490.4
BBB |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 398.5 | 396.8
Other |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | $ 7.7 | $ 8.7

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