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Investments in Unconsolidated Entities - White Mountain's Investments (Details) | Mar. 31, 2019 | Dec. 31, 2018 | Jan. 24, 2018
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MediaAlpha |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 48.30% |  |
DavidShield & PassportCard |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 50.00% | 50.00% |
PassportCard |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 25.00% | 25.00% | 50.00%
DavidShield |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 50.00% | 50.00% | 50.00%
Kudu |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 49.50% | 49.50% |
durchblicker |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 45.00% | 45.00% |
Compare.com |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 18.40% | 18.40% |
Tuckerman Capital Fund III, L.P. |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 16.00% | 18.50% |
White Mountains | DavidShield |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 25.00% | 25.00% |

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