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Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 6 Months Ended | 10 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Jun. 30, 2013 | Nov. 05, 2015 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax | $ 7.1 | $ 3.9 | $ 6.8 | $ 7.3 | $ 12.3 | $ 7.0 | $ 12.5 | $ 13.8 |  |  | $ 25.1 | $ 45.6 | $ 36.6
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  |  |  | (39.2) | 81.2 | (106.4)
Carrying value at the end of the period | 0.0 |  |  |  | 414.4 |  |  |  |  | $ 392.8 | 0.0 | 414.4 |
Tax benefit (expense) | $ 1.5 | $ 1.6 | $ 2.2 | $ (4.6) | 26.2 | $ 4.7 | $ (7.1) | $ (9.0) |  |  | $ 0.7 | 14.8 | (32.6)
Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  |  | 25.1 |  | 47.0 | 37.8
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  |  | (39.2) |  | 81.2 | (106.4)
Dividends received |  |  |  |  |  |  |  |  |  | (16.9) |  | (34.1) | (6.4)
Increase (decrease) in value of warrants |  |  |  |  |  |  |  |  |  |  |  |  | 10.8
Carrying value at the end of the period |  |  |  |  | 411.4 |  |  |  |  | 392.8 |  | 411.4 | 317.3
Unrealized (losses) gains from Symetra’s fixed maturity portfolio |  |  |  |  | 37.6 |  |  |  |  | (1.6) |  | 37.6 |
Exercise of warrants |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  |  | 25.1 |  | 47.0 | 37.8
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  |  | (39.2) |  | 81.2 | (106.4)
Carrying value of a distribution |  |  |  |  |  |  |  |  |  | 12.4 |  |  |
Dividends received |  |  |  |  |  |  |  |  |  | (16.9) |  | (34.1) | (6.4)
Increase (decrease) in value of warrants |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Carrying value at the end of the period |  |  |  |  | 411.4 |  |  |  |  | 392.8 |  | 411.4 | 317.3
Loss from dilutive effect resulting from dividend and issuance of restricted shares |  |  |  |  |  |  |  |  |  | (0.1) |  | (0.1) | 0.2
Aggregate value of White Mountains common shares of Symetra at quoted market price |  |  |  |  |  |  |  |  |  | $ 651.2 |  |  |
Share Price |  |  |  |  |  |  |  |  |  | $ 31.67 |  |  |
Exercise of warrants |  |  |  |  |  |  |  |  |  |  |  |  | 41.1
Common Stock | White Mountains |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  |  | $ (1.6) |  | 37.6 | (43.6)
Warrants |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in value of warrants |  |  |  |  |  |  |  |  | $ 10.8 |  |  |  |
Warrants | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  |  | 0.0 |  | 0.0 | 0.0
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  |  | 0.0 |  | 0.0 | 0.0
Dividends received |  |  |  |  |  |  |  |  |  | 0.0 |  | 0.0 | 0.0
Increase (decrease) in value of warrants |  |  |  |  |  |  |  |  |  |  |  |  | 10.8
Carrying value at the end of the period |  |  |  |  | $ 0.0 |  |  |  |  | 0.0 |  | 0.0 | $ 0.0
Share Price |  |  |  |  |  |  |  |  |  |  |  |  | $ 11.49
Exercise of warrants |  |  |  |  |  |  |  |  |  |  |  |  | $ (41.1)
equity in earnings of unconsolidated affiliates [Member] | Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Tax benefit (expense) |  |  |  |  |  |  |  |  |  | (1.6) |  | (3.3) | (2.8)
Net unrealized gains (losses) of unconsolidated affiliates [Member] | Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Tax benefit (expense) |  |  |  |  |  |  |  |  |  | 2.9 |  | (5.9) | 8.3
Net unrealized gains (losses) of unconsolidated affiliates [Member] | Common Stock | White Mountains |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Tax benefit (expense) |  |  |  |  |  |  |  |  |  | 0.2 |  | (2.7) | 3.2
impairment of equity in earnings [Member] | equity in earnings of unconsolidated affiliates [Member] | Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 3.0
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 gains (losses) on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  |  | 9.4 |  | 12.7 | $ 11.8
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 |  | $ 3.0 |

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