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Significant Transactions (Tables) | 9 Months Ended
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Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract] |
Reconciliation to the increase in book value | A reconciliation of the gain reported in discontinued operations to the impact to White Mountains’s book value is as follows:
Gain from sale of Tranzact reported in discontinued operations |  | $ | 51.9 |
Add back reclassification from continuing operations for the release of a tax valuation allowance |  | 30.2 |
Increase to White Mountains’s book value from sale of Tranzact |  | $ | 82.1 |

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