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Investments in Unconsolidated Entities - White Mountain's Investments (Details) | Jun. 30, 2019 | Jun. 03, 2019 | Apr. 04, 2019 | Apr. 03, 2019 | Dec. 31, 2018 | Jan. 24, 2018
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DavidShield & PassportCard |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership interest (as a percent) | 50.00% |  |  |  | 50.00% |
MediaAlpha |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership interest (as a percent) | 48.30% |  |  |  |  |
durchblicker |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership interest (as a percent) | 45.00% |  |  |  | 45.00% |
Elementum Holdings, L.P. |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership interest (as a percent) | 30.00% | 30.00% |  |  |  |
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) | 16.20% |  |  |  | 18.50% |
Kudu |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership interest (as a percent) |  |  | 99.10% | 49.50% | 49.50% |
DavidShield |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership interest (as a percent) | 50.00% |  |  |  | 50.00% | 50.00%
PassportCard |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership interest (as a percent) | 25.00% |  |  |  | 25.00% | 50.00%
White Mountains | DavidShield |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership interest (as a percent) | 25.00% |  |  |  | 25.00% |

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