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Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions | Jun. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Kudu investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | $ 315.6 | $ 266.5
MediaAlpha |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 225.0 | 180.0
DavidShield & PassportCard |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 95.0 | 90.0
Elementum Holdings, L.P. |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 55.1 | 55.1
Other unconsolidated entities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 31.9 | 33.7
Total unconsolidated entities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 722.6 | 625.3
Hedge and private equity funds included in other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 105.9 | 161.1
Insurance-linked securities funds |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 52.1 | 41.2
Private Debt Instrument |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 17.5 | 28.7
Noblr [Member] |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 4.6 | 5.0
White Mountains |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | $ 898.1 | $ 856.3

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