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Significant Transactions (Tables) | 12 Months Ended
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Dec. 31, 2017
Significant Transactions [Abstract] |
Reconciliation to the increase in book value | The following table presents a reconciliation of the gain reported in discontinued operations to the impact to White Mountains’s book value:
Millions |  | Year ended December 31, 2016
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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