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Investments in Unconsolidated Entities - White Mountain's Investments (Details) | Sep. 30, 2019 | May 31, 2019 | Dec. 31, 2018
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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%
JAM Partners L.P. |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 11.10% |  | 12.30%
Kudu |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) |  |  | 49.50%
Minimum | Tuckerman Capital Funds |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 18.80% |  | 18.50%
Minimum | Enlightenment Capital Funds |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 10.00% |  | 10.30%
Minimum | Kudu Investments |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 3.20% |  |
Maximum | Tuckerman Capital Funds |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 62.40% |  | 62.30%
Maximum | Enlightenment Capital Funds |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 36.50% |  | 36.50%
Maximum | Kudu Investments |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Ownership interest (as a percent) | 30.00% |  |

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