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Investment Securities - Debt securities issued by corporation (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Dec. 31, 2018
--- | --- | --- | --- | ---
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | $ 2,533.4 | $ 2,746.0 | $ 2,569.4 | $ 2,328.7
Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 528.7 | 467.2 |  |
AAA | Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 11.2 | 9.5 |  |
AA | Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 58.2 | 73.9 |  |
A | Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 307.7 | 288.5 |  |
BBB | Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 150.0 | 95.3 |  |
BB | Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | $ 1.6 | $ 0.0 |  |

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