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Financial Statement Revisions - Balance Sheet (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Total assets | $ 3,659.2 | $ 6,520.2 |
Investments | 3,380.7 | 2,714.4 |
Goodwill | 25.9 | 25.9 | $ 18.6
Intangible assets | 71.0 | 43.5 |
Total liabilities | 298.4 | 2,804.2 |
Liabilities held for sale |  | 2,569.3 |
White Mountains’s common shares | 3.8 | 4.6 |
Paid-in surplus | 666.8 | 806.1 |
Retained earnings | 2,823.2 | 2,776.6 |
Accumulated other comprehensive income, net of tax |  | (4.6) |
Total White Mountains’s common shareholders’ equity | 3,492.5 | 3,582.7 |
Non-controlling interests | (131.7) | 133.3 |
Total equity | 3,360.8 | 3,716.0 |
Total liabilities and equity | 3,659.2 | 6,520.2 |
Previously Reported |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Total assets |  | 6,544.7 |
Total liabilities |  | 2,807.5 |
White Mountains’s common shares |  | 4.6 |
Paid-in surplus |  | 806.1 |
Retained earnings |  | 2,797.2 |
Accumulated other comprehensive income, net of tax |  | (4.6) |
Total White Mountains’s common shareholders’ equity |  | 3,603.3 |
Non-controlling interests |  | 133.9 |
Total equity |  | 3,737.2 |
Total liabilities and equity |  | 6,544.7 |
Adjustment |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Total assets |  | (24.5) |
Total liabilities |  | (3.3) |
White Mountains’s common shares |  | 0.0 |
Paid-in surplus |  | 0.0 |
Retained earnings |  | (20.6) |
Accumulated other comprehensive income, net of tax |  | 0.0 |
Total White Mountains’s common shareholders’ equity |  | (20.6) |
Non-controlling interests |  | (0.6) |
Total equity |  | (21.2) |
Total liabilities and equity |  | (24.5) |
HG Global BAM |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Total assets | 747.4 | 677.6 |
Investments | 693.4 | 629.7 |
Total liabilities | 167.0 | 108.9 |
Accrued incentive compensation | 18.2 | 16.6 |
Other liabilities | 11.4 | 9.4 |
HG Global BAM | Previously Reported |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Total assets |  | 677.6 |
Total liabilities |  | 108.9 |
HG Global BAM | Adjustment |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Total assets |  | 0.0 |
Total liabilities |  | 0.0 |
MediaAlpha |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Total assets | 96.5 | 57.6 |
Total liabilities | 59.8 | 28.3 |
Accrued incentive compensation | 2.0 | 0.0 |
Other liabilities | 2.4 | 2.6 |
MediaAlpha | Previously Reported |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Total assets |  | 57.6 |
Total liabilities |  | 28.3 |
MediaAlpha | Adjustment |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Total assets |  | 0.0 |
Total liabilities |  | 0.0 |
Other |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Total assets | 2,815.3 | 5,785.0 |
Investments | 2,687.3 | 2,084.7 |
Goodwill |  | 7.6 |
Intangible assets |  | 1.0 |
Other intangible assets |  | 0.0 |
Other assets |  | 85.3 |
Assets held for sale | 3.3 | 3,606.4 |
Total liabilities | 71.6 | 2,667.0 |
Accrued incentive compensation | 60.6 | 79.1 |
Other liabilities | 11.0 | 18.6 |
Liabilities held for sale | $ 0.0 | 2,569.3 |
Other | Previously Reported |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Total assets |  | 5,809.5 |
Investments |  | 2,084.7 |
Goodwill |  | 13.4 |
Intangible assets |  | 4.7 |
Other intangible assets |  | 14.8 |
Other assets |  | 85.5 |
Assets held for sale |  | 3,606.4 |
Total liabilities |  | 2,670.3 |
Accrued incentive compensation |  | 79.1 |
Other liabilities |  | 21.9 |
Liabilities held for sale |  | 2,569.3 |
Other | Adjustment |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |
Total assets |  | (24.5) |
Investments |  | 0.0 |
Goodwill |  | (5.8) |
Intangible assets |  | (3.7) |
Other intangible assets |  | (14.8) |
Other assets |  | (0.2) |
Assets held for sale |  | 0.0 |
Total liabilities |  | (3.3) |
Accrued incentive compensation |  | 0.0 |
Other liabilities |  | (3.3) |
Liabilities held for sale |  | $ 0.0 |

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