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Investment Securities - Other long-term investments (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019
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Debt and Equity Securities, FV-NI [Line Items] |  |
Other Long-term Investments | $ 1,171.2 | $ 856.3
Marketing Technology (MediaAlpha) |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Long-term Investments | 475.0 | 180.0
Asset Management (Kudu) |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Long-term Investments | 325.5 | 266.5
PassportCard/DavidShield |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Long-term Investments | 95.0 | 90.0
Elementum Holdings, L.P. |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Long-term Investments | 55.1 | 55.1
Other unconsolidated entities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Long-term Investments | 31.8 | 33.7
Total unconsolidated entities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Long-term Investments | 982.4 | 625.3
Private equity funds and hedge funds |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Long-term Investments | 118.2 | 161.1
Insurance-linked securities funds |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Long-term Investments | 52.7 | 41.2
Private debt instruments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Long-term Investments | 17.9 | 28.7
Private debt investments | $ 5.1 | $ 5.0

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