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Segment Information (Assets) (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2012
Segment reporting information |  |  |
Total investments | $ 3,380.7 | $ 2,714.4 |
Assets held for sale |  | 3,606.4 |
Assets | 3,659.2 | 6,520.2 |
Liabilities held for sale |  | 2,569.3 |
Liabilities | 298.4 | 2,804.2 |
White Mountains’ common shareholders’ equity | 3,492.5 | 3,582.7 |
Non-controlling interests | (131.7) | 133.3 |
HG Global BAM |  |  |
Segment reporting information |  |  |
Total investments | 693.4 | 629.7 |
Assets held for sale |  | 0.0 |
Assets | 747.4 | 677.6 |
Liabilities held for sale |  | 0.0 |
Liabilities | 394.9 | 289.1 |
White Mountains’ common shareholders’ equity | 499.8 | 522.8 |
Non-controlling interests | (147.3) | (134.3) |
BAM |  |  |
Segment reporting information |  |  |
Surplus notes | 499.0 | 300.0 |
Accrued interest on surplus notes | 126.0 | 108.0 |
MediaAlpha |  |  |
Segment reporting information |  |  |
Total investments | 0.0 | 0.0 |
Assets held for sale |  | 0.0 |
Assets | 96.5 | 57.6 |
Liabilities held for sale |  | 0.0 |
Liabilities | 59.8 | 28.3 |
White Mountains’ common shareholders’ equity | 23.6 | 17.6 |
Non-controlling interests | 13.1 | 11.7 |
Other Operations |  |  |
Segment reporting information |  |  |
Total investments | 2,687.3 | 2,084.7 |
Assets held for sale |  | 0.0 |
Assets | 3,039.9 | 2,358.8 |
Liabilities held for sale |  | 0.0 |
Liabilities | 71.6 | 97.7 |
White Mountains’ common shareholders’ equity | 2,965.8 | 2,258.1 |
Non-controlling interests | 2.5 | 3.0 |
Total Capital Contributed to Subsidiary | 727.7 | 703.0 |
Eliminations |  |  |
Segment reporting information |  |  |
Assets | (227.9) | (180.2) |
Liabilities | (227.9) | (180.2) |
Discontinued Operations, Held-for-sale |  |  |
Segment reporting information |  |  |
Total investments | 0.0 | 0.0 |
Assets held for sale |  | 3,606.4 |
Assets | 3.3 | 3,606.4 |
Liabilities held for sale |  | 2,569.3 |
Liabilities | 0.0 | 2,569.3 |
White Mountains’ common shareholders’ equity | 3.3 | 784.2 |
Non-controlling interests | 0.0 | 252.9 |
Continuing Operations |  |  |
Segment reporting information |  |  |
Non-controlling interests | (131.7) | 133.3 |
HG Global |  |  |
Segment reporting information |  |  |
Non-controlling interests | 15.9 | 16.6 |
Surplus notes |  |  | $ 503.0
HG Global | BAM |  |  |
Segment reporting information |  |  |
Surplus notes | 499.0 | 503.0 |
Preferred Dividends Payable | HG Global | Other Operations |  |  |
Segment reporting information |  |  |
Accounts Payable and Accrued Liabilities | $ 227.9 | $ 180.2 |

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