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Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions | Jun. 30, 2019 | Dec. 31, 2018
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Kudu Investment Management, LLC |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | $ 142.2 | $ 0.0
MediaAlpha |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 114.7 | 0.0
DavidShield & PassportCard |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 75.0 | 75.0
Elementum Holdings, L.P. |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 55.1 | 0.0
Other unconsolidated entities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 39.1 | 60.0
Kudu |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 0.0 | 30.7
Total unconsolidated entities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 426.1 | 165.7
Hedge and private equity funds included in other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 166.1 | 146.1
Insurance-linked securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 40.3 | 0.0
Other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 31.9 | 13.8
White Mountains |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | $ 664.4 | $ 325.6

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