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Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018
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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
Kudu Investment Management, LLC |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 71.7 | 30.7
Other unconsolidated entities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 54.4 | 60.0
Total unconsolidated entities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 315.8 | 165.7
Hedge and private equity funds included in other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 150.7 | 146.1
Other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | 31.1 | 13.8
White Mountains |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other Investments | $ 497.6 | $ 325.6

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- Definition Amount of investments classified as other. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(f,g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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