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Investments in Unconsolidated Affiliates Investments in Unconsolidated Affiliates (White Mountain's Investments) (Details) | Dec. 31, 2017
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Compare.com |
Schedule of Equity Method Investments [Line Items] |
Ownership interest (as a percent) | 22.00%
durchblicker |
Schedule of Equity Method Investments [Line Items] |
Ownership interest (as a percent) | 45.00%
OneTitle |
Schedule of Equity Method Investments [Line Items] |
Ownership interest (as a percent) | 20.00%
PassportCard |
Schedule of Equity Method Investments [Line Items] |
Ownership interest (as a percent) | 50.00%
Tuckerman Capital Fund III |
Schedule of Equity Method Investments [Line Items] |
Ownership interest (as a percent) | 21.00%

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