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Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) - USD ($) $ / shares in Units, $ in Millions |  |  | 3 Months Ended | 10 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Nov. 05, 2015 | Dec. 31, 2014 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Nov. 05, 2015 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ 7.1 | $ 3.9 | $ 6.8 | $ 7.3 |  | $ 0.0 | $ 25.1 | $ 45.6
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  |  |  |  | 0.0 | (39.2) | 81.2
Income Tax Expense (Benefit) |  |  | $ 23.0 | $ 6.7 | $ 6.0 | $ 9.7 | $ 1.0 | $ 1.6 | $ 2.2 | $ (4.6) |  | $ 45.4 | $ 0.2 | 14.8
Common shares, issued shares |  |  | 4,563,814 |  |  |  | 5,623,735 |  |  |  |  | 4,563,814 | 5,623,735 |
Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  |  |  | $ 25.1 |  |  |
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  |  |  | (39.2) |  |  |
Carrying value of a distribution |  |  |  |  |  |  |  |  |  |  | 12.4 |  |  |
Dividends received |  |  |  |  |  |  |  |  |  |  | (16.9) |  |  |
Carrying value at the end of the period | $ 392.8 | $ 411.4 |  |  |  |  |  |  |  |  | 392.8 |  |  | 411.4
Loss from dilutive effect resulting from dividend and issuance of restricted shares |  |  |  |  |  |  |  |  |  |  | (0.1) |  |  |
Aggregate value of White Mountains common shares of Symetra at quoted market price | $ 651.2 |  |  |  |  |  |  |  |  |  | $ 651.2 |  |  |
Share Price | $ 31.67 |  |  |  |  |  |  |  |  |  | $ 31.67 |  |  |
Common Stock | White Mountains |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates | $ (1.6) | $ 37.6 |  |  |  |  |  |  |  |  |  |  |  |
equity in earnings of unconsolidated affiliates [Member] | Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  |  |  | $ (1.6) |  |  |
Net unrealized gains (losses) of unconsolidated affiliates [Member] | Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  |  |  | 2.9 |  |  |
Net unrealized gains (losses) of unconsolidated affiliates [Member] | Common Stock | White Mountains |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  |  |  | 0.2 |  |  | $ (2.7)
impairment of net unrealized gains (losses) [Member] | Net unrealized gains (losses) of unconsolidated affiliates [Member] | Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in pre-tax unrealized investment (losses) gains on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  |  |  | 9.4 |  |  |
Amortization of common share basis difference [Member] | Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  |  |  | $ 2.3 |  |  |
Prospector |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common shares, issued shares |  |  |  |  |  |  | 513,500 |  |  |  |  |  | 513,500 |

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