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Segment Information (Assets) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
--- | --- | --- | --- | ---
Segment reporting information |  |  |  |  |  |
Total investments | $ 7,192.6 |  | $ 7,278.1 |  |  |
Reinsurance recoverables, gross | 453.5 |  | 447.0 |  |  |
Assets held for sale | 1,880.1 |  | 2,226.8 |  |  |
Assets | 12,144.3 |  | 12,895.4 |  |  |
Loss and loss adjustment expense reserves | 3,079.3 |  | 3,168.9 |  | 5,702.3 | 5,736.8
Liabilities held for sale | 1,880.1 |  | 2,226.8 |  |  |
Liabilities | 7,746.9 |  | 8,637.2 |  |  |
Total non-controlling interests | 491.9 |  | 526.4 |  |  |
White Mountains’ common shareholders’ equity | 3,905.5 |  | 3,731.8 |  |  |
OneBeacon |  |  |  |  |  |
Segment reporting information |  |  |  |  |  |
Total investments | 2,364.9 |  | 2,291.5 |  |  |
Reinsurance recoverables, gross | 89.9 |  | 110.7 |  |  |
Assets held for sale | 1,880.1 |  | 2,226.8 |  |  |
Assets | 5,191.3 |  | 5,382.3 |  |  |
Loss and loss adjustment expense reserves | 1,054.3 |  | 1,000.0 |  |  |
Liabilities held for sale | 1,880.1 |  | 2,226.8 |  |  |
Liabilities | 4,083.9 |  | 4,365.0 |  |  |
Total non-controlling interests | 276.8 |  | 254.2 |  |  |
White Mountains’ common shareholders’ equity | 830.6 |  | 763.1 |  |  |
Sirius Group |  |  |  |  |  |
Segment reporting information |  |  |  |  |  |
Total investments | 3,251.4 |  | 3,534.3 |  |  |
Reinsurance recoverables, gross | 363.6 | [1] | 336.3 |  |  |
Assets held for sale | 0 |  | 0 |  |  |
Assets | 5,124.1 |  | 5,962.0 |  |  |
Loss and loss adjustment expense reserves | 2,025.0 |  | 2,168.9 |  |  |
Liabilities held for sale | 0 |  | 0 |  |  |
Liabilities | 3,414.5 |  | 4,123.2 |  |  |
Total non-controlling interests | 250.0 |  | 279.1 |  |  |
White Mountains’ common shareholders’ equity | 1,459.6 |  | 1,559.7 |  |  |
HG Global |  |  |  |  |  |
Segment reporting information |  |  |  |  |  |
Total investments | 107.9 |  | 101.5 |  |  |
Reinsurance recoverables, gross | 0 |  | 0 |  |  |
Assets held for sale | 0 |  | 0 |  |  |
Assets | 675.0 |  | 623.6 |  |  |
Loss and loss adjustment expense reserves | 0 |  | 0 |  |  |
Liabilities held for sale | 0 |  | 0 |  |  |
Liabilities | 68.9 |  | 0.9 |  |  |
Total non-controlling interests | 16.6 |  | 16.6 |  |  |
White Mountains’ common shareholders’ equity | 589.5 |  | 606.2 |  |  |
BAM |  |  |  |  |  |
Segment reporting information |  |  |  |  |  |
Surplus notes | 503.0 |  |  |  |  |
Total investments | 468.6 |  | 472.4 |  |  |
Reinsurance recoverables, gross | 0 |  | 0 |  |  |
Assets held for sale | 0 |  | 0 |  |  |
Assets | (77.9) | [2] | (28.6) | [2] |  |
Loss and loss adjustment expense reserves | 0 |  | 0 |  |  |
Liabilities held for sale | 0 |  | 0 |  |  |
Liabilities | 19.7 |  | 7.4 |  |  |
Total non-controlling interests | (97.6) |  | (36.0) |  |  |
White Mountains’ common shareholders’ equity | 0 |  | 0 |  |  |
Accrued interest on surplus notes | 58.6 |  | 18.4 |  |  |
Other Operations |  |  |  |  |  |
Segment reporting information |  |  |  |  |  |
Total investments | 999.8 |  | 878.4 |  |  |
Reinsurance recoverables, gross | 0 |  | 0 |  |  |
Assets held for sale | 0 |  | 0 |  |  |
Assets | 1,231.8 |  | 956.1 |  |  |
Loss and loss adjustment expense reserves | 0 |  | 0 |  |  |
Liabilities held for sale | 0 |  | 0 |  |  |
Liabilities | 159.9 |  | 140.7 |  |  |
Total non-controlling interests | 46.1 |  | 12.5 |  |  |
White Mountains’ common shareholders’ equity | $ 1,025.8 |  | $ 802.8 |  |  |
[1] | Standard & Poor’s ratings as detailed above are: “AAA” (Extremely strong), “AA” (Very strong), “A” (Strong), and “BBB+” and “BBB” (Adequate).
[2] | BAM total assets reflect the elimination of $503.0 in surplus notes issued to HG Global and its subsidiaries, and $58.6 and $18.4 in accrued interest related to those surplus notes as of December 31, 2013 and 2012.

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